The context: the selection of a maximum value IT service portfolio is a major challenge to be faced by IT executives, during the Service Strategy phase of the IT service life cycle defined in ITIL. The problem: ITIL states that the value of an IT service should be estimated based on the improvement of customers' outcomes brought by the service. It is hard to apply this approach due to difficulties to access data related to customers' outcomes and to precisely isolate the influence of IT on the improvements of customers' performance. Our proposed solution: in this work, we propose the application of a formal business value model to quantify the business value of IT services, so as to pave the way for the definition of a maximum-value portfolio. The quantification method presented estimates the value of IT services through low-intrusion interactions between the IT Service provider and its customers. Validation: our approach was applied with success to the IT department of a multinational manufacturer. Results from the experiments enabled higher expected IT ROI through the use of a value-based portfolio definition process.