Workflows have generally been accepted as a means to model and support processes in complex organizations. The fact that these processes require robustness and clear semantics has generally been observed and has lead to the combination of workflow and transaction concepts. Many variations on this combination exist, leading to many approaches to transactional workflow support. No clear classification of these approaches has been developed, however, resulting in a badly understood field. To deal with this problem, we describe a clear taxonomy of transactional workflow models, based on the relation between workflow and transaction concepts. We show that the classes in the taxonomy can directly be related to specification language and architecture types for workflow and transaction management systems. We compare the classes with respect to their characteristics and place existing approaches in the taxonomy - thus offering a basis for analysis of transactional workflow support.