Whether social networks diffuse knowledge across firm boundaries has been the topic of much debate. To inform these theories, this article considers two questions. First, who has contacts across firm boundaries? And second, when do these relations diffuse knowledge? Our empirical evidence comes from a survey of 346 engineers in the wireless communication industry around Aalborg in Northern Denmark. Our analysis finds that social contact between these engineers is frequent and is used to diffuse knowledge that receivers find useful. More experienced engineers are more likely to receive valuable knowledge from their networks. These findings show that the long-term relationships, which are more likely based on trust and reputation, are also more likely to be a channel valuable knowledge. (c) 2004 Elsevier B.V. All rights reserved.