Knowledge intensive services (KIS) and, in particular, R&D services contribute significantly to innovation in firms. The objective of this paper is to find out which characteristics of firms explain the acquisition of R&D services and to analyse whether there are differences depending on the typology of the supplier (universities, technology centres and consulting firms). Three main conclusions emerge from the econometric estimations. Firstly, the results show that size and age matter in the decision to buy R&D services, but these characteristics of firms do not have any particular influence in the decision to choose a specific supplier. Secondly, our results are consistent with the relevance that the literature gives to human capital in absorbing external knowledge. The variables used to control for human skills have a positive effect on the decision to buy R&D services. On the contrary, the estimates of other variables that capture internal knowledge base suggest that there is a substitution process between internal R&D activities and acquiring R&D services. Thirdly, innovation policy has a significant influence on the decision to acquire R&D services.