A growing international literature advocates the importance of trust and co-operation in tax administration compared to the more traditional ‘adversarial' approach. Yet, the global financial crisis has led to renewed interest in corporate tax planning and ‘unacceptable' tax avoidance with a focus on the role of intermediaries such as accounting firms. This article explores how developments in tax legislation are captured by companies and incorporated into their tax knowledge. We draw on prior literature in knowledge management, the role of accounting firms and tax administration and use a qualitative approach to investigate and describe the relationships between accounting firms, corporate taxpayers and revenue authorities, specifically the UK HM Revenue and Customs (HMRC). Our results show that these relationships can be described in the context of a tax knowledge market comprised of a knowledge seller, knowledge brokers, and knowledge buyers. Our findings have relevance not only for all three parties, in particular for the challenges facing knowledge brokers – but also to tax agencies and international organisations, who must strike the optimal balance between co-operative tax administration and traditional approaches buttressed by tax audit enforcement.