IT service organisations recognise the value of conducting regular process assessments for continual service improvement. However lack of transparency and substantial costs deter industry adoption. We propose that the use of the international standard for process assessment ISO/IEC 15504 offers a transparent approach to address this challenge. Moreover, efficiency can be realized by a Decision Support System (DSS) tool to automate data collection and process capability calculations. This paper details a Design Science Research project to develop a software-mediated process assessment (SMPA) approach based on ISO/IEC 15504, ISO/IEC 20000 and the IT Infrastructure Library (ITIL We discuss the architecture of the SMPA approach and the role of ISO/IEC 15504 in the approach. This work contributes to practice as it may help IT managers to self-assess their processes using a standard model. The SMPA approach can also support assessors who perform formal assessments.