Purpose – This study aims to conceptualise the role of knowledge transfer offices (KTOs) as knowledge brokers (KBs) and identify which factors are most significantly related with their performance for supporting public-private research organizations (PROs), testing the authors' hypothesis for the Spanish case. Design/methodology/approach – An empirical analysis is conducted based on data from RedOTRI 2008 annual report about 63 Spanish KTOs. A multiple lineal regression model is carried on each of the selected variables representative of KTOs' performance (number of priority patents, revenues from industry collaboration and number of spin-offs) in order to establish possible relationships with some factors related to the knowledge process that characterize KTOs' activity. Findings – A theoretical framework conceptualizing the KTOs' role as knowledge brokers is suggested. Factors positively influencing KTOs' performance are PRO's total annual expenses, the type of PRO, the KTO age, the existence of a science park, the explicit regulation of intellectual property rights, the number of specialized full-time staff of the KTO and the availability of a patent stock. Practical implications – The practical implication is the identification of those critical factors for the day-to-day operation of Spanish KTOs in their different ways of transferring knowledge, drawing managerial and organisational practices that may improve their performance. Originality/value – This paper provides two original contributions for literature on knowledge transfer: a theoretical framework for the conceptualisation of KTOs as KBs, and the categorisation and further analysis of factors closely related to the performance of KTOs. A set of managerial implications for a better improvement of such institutions is presented. ; Postprint (published version)