The graphical modelling of processes is of growing popularity and high relevance to organisations that seek to document, analyse and improve their business operations. This research investigates the phenomenon of continued user acceptance of the grammars that are used to build process models. It develops and tests a theory that can be used to explain and predict why users would opt to continue working with certain grammars in their process modelling efforts. This study builds on established theories, including the Technology Acceptance Model, Expectation-Confirmation Theory, Task-Technology Fit Theory and Representation Theory. These theories suggest that end users typically strive for tools that are useful and easy to use, which confirm their expectations through firsthand utility, and which match task requirements and individual abilities. Representation theory suggests that modelling grammars should be complete and clear in their capabilities to represent real-world domains. The research model has been designed by combining conceptual studies of acceptance and continuance theories with a representational analysis of the BPMN grammar, which is a recently ratified industry standard for process modelling and thereby of high practical relevance to process modelling practice. It further incorporates findings from nineteen semi-structured interviews with process modellers in Australia. The research model has been tested and validated by means of a web-based survey with 590 process modellers world-wide. This thesis contributes to the body of knowledge in a number of ways: First, it presents an empirically validated model of the factors determining a user's intention to continue using a process modelling grammar. Second, it measures the impact that grammar characteristics as well as user and task characteristics have on user evaluations of a process modelling grammar. Third, it presents empirical evidence on the consequences that perceived representational deficiencies entail on user perceptions of a process modelling grammar.