The tax reforms in the transformation countries of Eastern Europe were shaped by economic advisors. Significant is the tax policy advice from the International Monetary Fund (IMF). The author raises the following fundamental questions: Should the IMF even provide tax policy advice? What does modern tax policy advice look like? What distinguishes "good" advice from "bad" advice? By what criteria should economic policy recommendations be evaluated? These questions are examined through a case study of Croatia and Bosnia-Herzegovina. After a comprehensive presentation of the tax reforms there, the work of the IMF is subjected to a normative-economic analysis and critically examined in light of recent institutional economic approaches. The work also contributes to the theory and practice of taxation.