In many organisations the work of delivering IT services in order to maintain and improve the IT portfolio is a complex task. Different models for IT service management have been developed and they are widely in use. An ITSM model is a way of institutionalising processes, activities and roles of IT service delivery. This research studies the use of an ITSM model (an adaptation of ITIL) in a municipal IT department. A qualitative case study has been conducted. The adapted ITSM model has been investigated and compared with several instances of IT service deliveries. Six cases of IT service deliveries (of both failure and success character) have been studied. This inquiry uses a storytelling approach and a combination of different data sources (workshop discussions, interviews, documents, records and the functionality of used IT artefacts). The analysis of these cases is conducted in a theory-informed way. A conceptual model has been developed describing an ITSM model as an organisational institution manifested in different institutional carriers. Based on rich empirical data we have clarified positive and negative effects of both ITSM model compliance and deviation through improvisation. A dialectical model is developed showing a possible synthesis of ITSM model compliance and improvisation.