A socio-instrumental service modelling approach is presented through a tax declaration case study. Three different service alternatives have been investigated (two paper form alternatives and one e-service). These service alternatives have been studied through service interaction modelling, contextual service definitions and service pattern analysis. Service effects have been identified at both service parties; the service provider (the Tax Agency) and the service clients (companies). These service effects were dependent on affordances of services and service pre-conditions. These affordances can be of both social and technical-instrumental character. This study has contributed with service modelling methods and a new socio-instrumental conceptualisation of services. Important new notions are: reciprocal service effect, interdependence of services and service pre-conditions, unintended service affordances, reciprocal facilitation through service interaction. These concepts lead to the new concept of co-service.