Enterprise Systems (ES) are comprehensive off-the-shelf packages that have to be configured to suit the requirements of an organization. Most ES solutions provide reference models that describe the functionality and structure of the system. However, these models do not capture the potential configuration alternatives. This paper discusses the shortcomings of current reference modelling languages using Event-Driven Process Chains (EPCs) as an example. We propose Configurable EPCs (C-EPCs) as an extended reference modelling language which allows capturing the core configuration patterns. A formalization of this language as well as examples for typical configurations are provided. A program of further research including the identification of a comprehensive list of configuration patterns, deriving possible notations for reference model configurations and testing the quality of these proposed extensions in experiments and focus groups is presented.