A major limitation of current workflow management systems appears in (1) their lack of support for flexible workflows whose structures can be modified dynamically during the execution and (2) in their failure in dealing, efficiently, with the most advanced workflow patterns. In this paper, we propose a new model which we call recursive ECATNets (RECATNets) to model workflow processes with dynamic structure and, particularly, to handle the most complex workflow patterns, in a concise way. The RECATNets extend classical ECATNets (Extended Concurrent Algebraic Term Nets) with the recursion concept firstly introduced in the recursive Petri nets. We define the semantics of RECATNets in the conditional rewriting logic framework. Rewriting logic is a true concurrency and operational semantics which allows rapid prototyping using rewriting techniques and the system Maude in particular.