This paper examines the effect of relational factors on knowledge transfer within strategic buyer–supplier exchange. Prior research examining inter-firm knowledge transfer has focused almost exclusively on horizontal forms of governance such as strategic alliances and joint ventures, whilst research on vertical forms, such as buyer–supplier relationships, is limited. We test the effect of four important relational properties: cooperation, trust, relationship duration and supplier performance. Quantitative data, gathered from 104 UK manufacturing firms in eight industry sectors, are used to analyse the hypothesized relationships through a moderated hierarchical regression model. Our study provides support for the importance of considering relational factors in the transfer of knowledge at the inter-organizational level. In particular, the results indicate that knowledge transfer is positively influenced by the extent of cooperation, but that this relationship is moderated by the level of trust and the performance of the supplier firm. Managerial implications for these findings and future directions for research are then offered.