UML activity diagrams are a commonly used notation for modelling business processes in the field of both workflow automation and requirements engineering. In this paper, we present a novel precise style for this notation. Further, the effectiveness of this style has been investigated in the context of the modelling of business processes through a controlled experiment conducted with master students in Computer Science at the Free University of Bolzano-Bozen. The results indicate that the subjects achieved a significantly better comprehension level when business processes are modelled using the precise style with respect to a “lighter” variant, with no significant impact on the effort to accomplish the tasks