We uncover a “paradox of formal appropriability mechanisms” in the case of knowledge-intensive business services (KIBS) firms. Despite evidence that KIBS firms do not typically consider formal appropriability mechanisms, such as patents, to be central mechanisms for capturing value from innovation, we show that they are nevertheless important for their innovation collaboration. Drawing on an original survey of publicly-traded UK and US KIBS firms, we find a significant positive association between the importance of innovation collaboration and the importance of formal appropriability mechanisms. We interrogate the evidence for clients, as they are the most important partners for innovation collaboration. We find that the importance of innovation collaboration with clients goes hand-in-hand with the importance of formal appropriability mechanisms, although a negative relation appears when firms assign very high importance to formal appropriability mechanisms. Thus, modest levels of emphasis on formal appropriability mechanisms may prevent conflicts over ownership of jointly developed knowledge assets and knowledge leakages, while also avoiding the possibly negative effects of overly strict controls by legal departments on innovation collaboration. As well as exploring formal appropriability mechanisms, we also investigate the relationship between contractual and strategic appropriability mechanisms and innovation collaboration for KIBS firms.