Why did New Zealand adopt one of the most neoliberal tax systems in the world, whereas Ireland pursued a heterodox tax policy of low rates, deep deductions, and distortionary tax incentives? The diverging tax policy trajectories of these two small liberal market economies since 1980 are not well accounted for by conventional ideational, partisan, or political-institutional explanations. The article argues that the varying degree of neoliberal reform is better understood as the result of differences in the institutionalization of economic knowledge within the state. Distinct administrative institutions in New Zealand and Ireland gave rise to profound differences in the identities, expertise, economic ideas, and policy advice approach of tax policy bureaucrats, which had a major impact on tax policymaking in the two countries.