Evidence-based policy making has been advocated by liberal democracies around the world. To date, this approach has principally been pursued through ‘rationalist’ linear-technocratic decision making methods which assume that policy problems are tractable and that experts can provide adequate and impartial advice to government. Using case-studies in Queensland, Australia and the UK, this research utilises a comparative political analysis alongside the ‘knowledge systems’ framework first proposed by Cash et al. (2002) to understand how norms, values and prevailing politics influence evidence development and use for adaptation policy. In Queensland, the evidence-based mandate has been weakened by prevailing politics, even though policy makers still seek to develop a business-case, for which climate science is often perceived to be incompatible. In the UK by contrast, evidence-based policy is enshrined in the Climate Change Act (2008), yet how evidence has been developed under this mandate raises important questions about the extent to which it can ever be considered apolitical.