Currently many different modeling languages are used for workflow definitions in BPM systems. Authors of this paper analyze the two most popular graphical languages, with highest possibility of wide practical usage - UML Activity diagrams (AD) and Business Process Modeling Notation (BPMN). The necessary in practice workflow aspects are briefly discussed, and on this basis a natural AD profile is proposed, which covers all of them. A functionally equivalent BPMN subset is also selected. The semantics of both languages in the context of process execution (namely, mapping to BPEL) is also analyzed in the paper. By analyzing AD and BPMN metamodels, authors conclude that an exact transformation from AD to BPMN is not trivial even for the selected subset, though these languages are considered to be similar. Authors show how this transformation could be defined in the MOLA transformation language.