Abstract Policy evaluations can be set up for multiple purposes including accountability, policy learning and policy planning. The question is, however, how these purposes square with politics itself. To date, there is little knowledge on how government ministers present the rationale of evaluations. This article is the first to provide a diachronic study of discourse about evaluation purposes and encompass a wide range of policy fields. We present an analysis of evaluation announcements in so-called ministerial policy notes issued between 1999 and 2019 by the Flemish government in Belgium. The research fine-tunes available evidence on catalysts for conducting evaluations. The Flemish public sector turns out to be a strong case where New Public Management brought policy evaluation onto the agenda, but this has not resulted in a prominent focus on accountability-oriented evaluations. We further show that policy fields display different evaluation cultures, albeit more in terms of the volume of evaluation demand than in terms of preferences for particular evaluation purposes.