While in the area of relational databases interoperability is ensured by common communication protocols (e.g. ODBC/JDBC using SQL), Content Based Image Retrieval Systems (CBIRS) and other multimedia retrieval systems are lacking both a common query language and a common communication protocol. Besides its obvious short term convenience, interoperability of systems is crucial for the exchange and analysis of user data. In this paper, we present and describe an extensible XML-based query markup language, called MRML (Multimedia Retrieval Markup Language). MRML is primarily designed so as to ensure interoperability between different content based multimedia retrieval systems. Further, MRML allows researchers to preserve their freedom in extending their system as needed. MRML encapsulates multimedia queries in a way that enables multimedia (MM) query languages, MM content descriptions, MM query engines, and MM user interfaces to grow independently from each other, reaching a maximum of interoperability while ensuring a maximum of freedom for the developer. For benefiting from this, only a few simple design principles have to be respected when extending MRML for one's private needs. The design of extensions within the MRML framework will be described in detail in the paper. MRML has been implemented and tested for the CBIRS Viper, using the user interface SnakeCharmer. Both are under GNU Public License and can be downloaded at http://viper.unige.ch/download/