Index: include/exiv2/properties.hpp
===================================================================
--- include/exiv2/properties.hpp	(revision 4307)
+++ include/exiv2/properties.hpp	(working copy)
@@ -38,6 +38,7 @@
 #include "types.hpp"
 #include "metadatum.hpp"
 #include "tags.hpp"
+#include "rwlock.hpp"
 
 // + standard includes
 #include <string>
@@ -107,6 +108,11 @@
         //! Prevent assignment: not implemented.
         XmpProperties& operator=(const XmpProperties& rhs);
 
+      private:  
+        static const XmpNsInfo* nsInfoUnsafe(const std::string& prefix);
+        static void unregisterNsUnsafe(const std::string& ns);
+        static const XmpNsInfo* lookupNsRegistryUnsafe(const XmpNsInfo::Prefix& prefix);
+        
     public:
         /*!
           @brief Return the title (label) of the property.
@@ -173,6 +179,7 @@
           @throw Error if no namespace is registered with \em prefix.
          */
         static const XmpNsInfo* nsInfo(const std::string& prefix);
+
         /*!
            @brief Return the (preferred) prefix for schema namespace \em ns.
            @param ns Schema namespace
@@ -205,6 +212,7 @@
           @note This invalidates XMP keys generated in this namespace.
          */
         static void unregisterNs(const std::string& ns);
+
         /*!
           @brief Unregister all custom namespaces.
 
@@ -223,7 +231,7 @@
 
         // DATA
         static NsRegistry nsRegistry_;          //!< Namespace registry
-
+        static Internal::RWLock rwLock_;
     }; // class XmpProperties
 
     /*!
Index: src/epsimage.cpp
===================================================================
--- src/epsimage.cpp	(revision 4307)
+++ src/epsimage.cpp	(working copy)
@@ -51,6 +51,7 @@
 namespace {
 
     using namespace Exiv2;
+    using Exiv2::byte;
 
     // signature of DOS EPS
     const std::string dosEpsSignature = "\xC5\xD0\xD3\xC6";
Index: src/preview.cpp
===================================================================
--- src/preview.cpp	(revision 4307)
+++ src/preview.cpp	(working copy)
@@ -47,6 +47,7 @@
 namespace {
 
     using namespace Exiv2;
+    using Exiv2::byte;
 
     /*!
       @brief Compare two preview images by number of pixels, if width and height
Index: src/properties.cpp
===================================================================
--- src/properties.cpp	(revision 4307)
+++ src/properties.cpp	(working copy)
@@ -39,6 +39,7 @@
 #include "metadatum.hpp"
 #include "i18n.h"                // NLS support.
 #include "xmp.hpp"
+#include "rwlock.hpp"
 
 #include <iostream>
 #include <iomanip>
@@ -2205,9 +2206,16 @@
     }
 
     XmpProperties::NsRegistry XmpProperties::nsRegistry_;
+    Internal::RWLock XmpProperties::rwLock_;
 
     const XmpNsInfo* XmpProperties::lookupNsRegistry(const XmpNsInfo::Prefix& prefix)
     {
+        Internal::ScopedReadLock srl(rwLock_);
+        return lookupNsRegistryUnsafe(prefix);
+    }
+
+    const XmpNsInfo* XmpProperties::lookupNsRegistryUnsafe(const XmpNsInfo::Prefix& prefix)
+    {
         for (NsRegistry::const_iterator i = nsRegistry_.begin();
              i != nsRegistry_.end(); ++i) {
             if (i->second == prefix) return &(i->second);
@@ -2218,11 +2226,13 @@
     void XmpProperties::registerNs(const std::string& ns,
                                    const std::string& prefix)
     {
+        Internal::ScopedWriteLock swl(rwLock_);
+        
         std::string ns2 = ns;
         if (   ns2.substr(ns2.size() - 1, 1) != "/"
             && ns2.substr(ns2.size() - 1, 1) != "#") ns2 += "/";
         // Check if there is already a registered namespace with this prefix
-        const XmpNsInfo* xnp = lookupNsRegistry(XmpNsInfo::Prefix(prefix));
+        const XmpNsInfo* xnp = lookupNsRegistryUnsafe(XmpNsInfo::Prefix(prefix));
         if (xnp) {
 #ifndef SUPPRESS_WARNINGS
             if (strcmp(xnp->ns_, ns2.c_str()) != 0) {
@@ -2230,7 +2240,7 @@
                             << xnp->ns_ << " to " << ns2 << "\n";
             }
 #endif
-            unregisterNs(xnp->ns_);
+            unregisterNsUnsafe(xnp->ns_);
         }
         // Allocated memory is freed when the namespace is unregistered.
         // Using malloc/free for better system compatibility in case
@@ -2249,6 +2259,12 @@
 
     void XmpProperties::unregisterNs(const std::string& ns)
     {
+        Internal::ScopedWriteLock swl(rwLock_);
+        unregisterNsUnsafe(ns);
+    }
+
+    void XmpProperties::unregisterNsUnsafe(const std::string& ns)
+    {
         NsRegistry::iterator i = nsRegistry_.find(ns);
         if (i != nsRegistry_.end()) {
             std::free(const_cast<char*>(i->second.prefix_));
@@ -2259,15 +2275,18 @@
 
     void XmpProperties::unregisterNs()
     {
+        Internal::ScopedWriteLock swl(rwLock_);
+        
         NsRegistry::iterator i = nsRegistry_.begin();
         while (i != nsRegistry_.end()) {
             NsRegistry::iterator kill = i++;
-            unregisterNs(kill->first);
+            unregisterNsUnsafe(kill->first);
         }
     }
 
     std::string XmpProperties::prefix(const std::string& ns)
     {
+        Internal::ScopedReadLock srl(rwLock_);
         std::string ns2 = ns;
         if (   ns2.substr(ns2.size() - 1, 1) != "/"
             && ns2.substr(ns2.size() - 1, 1) != "#") ns2 += "/";
@@ -2285,9 +2304,10 @@
 
     std::string XmpProperties::ns(const std::string& prefix)
     {
-        const XmpNsInfo* xn = lookupNsRegistry(XmpNsInfo::Prefix(prefix));
+        Internal::ScopedReadLock srl(rwLock_);
+        const XmpNsInfo* xn = lookupNsRegistryUnsafe(XmpNsInfo::Prefix(prefix));
         if (xn != 0) return xn->ns_;
-        return nsInfo(prefix)->ns_;
+        return nsInfoUnsafe(prefix)->ns_;
     }
 
     const char* XmpProperties::propertyTitle(const XmpKey& key)
@@ -2351,8 +2371,15 @@
 
     const XmpNsInfo* XmpProperties::nsInfo(const std::string& prefix)
     {
+        Internal::ScopedReadLock srl(rwLock_);
+        return nsInfoUnsafe(prefix);
+    }
+
+    const XmpNsInfo* XmpProperties::nsInfoUnsafe(const std::string& prefix)
+    {
+        Internal::ScopedReadLock srl(rwLock_);
         const XmpNsInfo::Prefix pf(prefix);
-        const XmpNsInfo* xn = lookupNsRegistry(pf);
+        const XmpNsInfo* xn = lookupNsRegistryUnsafe(pf);
         if (!xn) xn = find(xmpNsInfo, pf);
         if (!xn) throw Error(35, prefix);
         return xn;