Divergent requirements of customers limit the potential of information technology (IT) service providers to achieve economies of scale through the standardization of service agreements. Continuous change requests in ongoing IT-service relationships complicate matters even more. Mass customization strategies have successfully addressed similar challenges in industrial sectors by reusing, i.e. composing and adapting standardized modules. Transforming this strategy to IT-service management, we present an approach of reuse-based IT-service customization in order to increase both effectiveness and efficiency at the stages of initial service specification, customization of offerings, and continuous adjustment of ongoing service agreements. This is proposed to be achieved by adopting well-established reusemechanisms of reference information modeling. Their strict application in service agreement specification aims for enabling industrialized on-demand service contracting and provisioning. The approach has been developed and prototypically applied in close cooperation with IT-organizations.