By definition, the outsourcing of a function requires its prior internal realization. However the Williamsonian transaction cost theory primarily rests on a cost comparison of alternative governance structures. The goal of this research is to make it more dynamic thanks to the concept of transition costs. This concept has been made deeper. An empirical study has been conducted on real decisions (resulting in outsourcing or in-house maintaining) in educational publishers. It enriches this dynamic view through a clarification of the costs involved in outsourcing and through the questioning of the influence of ex-ante transaction costs.