Hailed as the basis for competitive advantage of contemporary firms, knowledge transfer has recently emerged as a key research topic in the organisation and management field. Despite wide recognition of the importance of social relations for effective knowledge transfer, there is little understanding of the micro-sociological foundations of this process, and even less understanding of the ways in which human resource management practices can support social relations conducive to knowledge transfer and sharing. The present study developed an integrated conceptual model with the aim to improve understanding of the mechanisms for and the conditions under which social relations can be transformed into useful, actionable knowledge. To test this model, quantitative data were gathered through a questionnaire survey of 135 knowledge workers from three Irish-based organisations. Qualitative data were also collected through semi-structured interviews with the human resource managers and knowledge managers of these organisations. The findings demonstrated that, at the interpersonal level of analysis, the effective transfer of knowledge hinges upon the extent to which individuals share a common lexicon for communication and trust each other, both professionally and personally. In particular, personal trust was found to be key to the transfer of tacit knowledge, thereby underlining the importance of positive affect as a criterion for the formation of productive knowledge exchange relations. In regard to the role of human resource management, it was found that employees’ perceptions of reciprocal task interdependence, job feedback, selective staffing, intensive socialisation, and relational-oriented training and development are related strongly to their perceptions of a social climate of teamwork and cooperation and, consequently, of knowledge sharing attitudes. Importantly, the effect of these practices was found to be mediated by employees’ perceptions of line managers’ support for knowledge sharing. The study concluded by suggesting the need for further integration of social relations into research on the role of human resource management practices in knowledge transfer and organisational learning.