Since the use of mobile devices spreads increasingly, mobile systems also play a major role for distributed business processes. In such scenarios, extending workflow management support to mobile systems offers potential to seamlessly integrate field staff into business processes, even if executing devices are disconnected from the company's server. However, the heterogeneity of current mobile systems still requires complex device-specific descriptions of user interfaces to integrate manual tasks. Therefore, this paper presents an abstract and modality-independent description model to support the development and execution of interactive mobile workflows and a corresponding prototype realization based on a service-oriented execution module.