Recent studies suggest that service definition is an ongoing process, with service characteristics being temporarily stabilised and destabilised through buyer-service provider interactions. However, little is still known about how and, in particular, why services are (re)defined during the sourcing process, e.g., in terms of their intended outputs, how they are performed (processes), the inputs required and their economic outcomes. This paper addresses this gap as follows: (a) by examining in detail how service definitions are (de)stabilised during the sourcing process and (b) by identifying five categories of influencing factors (i.e., sourcing capability, supplier expertise reliance, complexity, relationship continuity and adaptive interactions) and developing propositions regarding their stabilising/destabilising impact on different service definition aspects. The study contributes to theory development on service definition dynamics and emphasises the positive effects of revisiting service specifications both pre- and post-contract. It also offers an extension of the theory of service definition methods by stressing their dynamic deployment during the sourcing process.