Gender mainstreaming is the main strategy for achieving gender equality in Sweden. With gender mainstreaming, a gender equality perspective is to be included in all areas, at all levels and all parts of the policy process. The strategy is considered to have a transformative potential, and the ability to reveal and challenge the genderedness of institutions. However, it has been criticized for being technocratic and not leading to the intended transformative effects. In this thesis, the institutionalization of gender mainstreaming is examined using a qualitative method based on most-likely cases: two Swedish municipalities. The aim of the thesis is twofold – to examine the strategy’s level of institutionalization, and to examine in what way it has been institutionalized in relation to the transformative potential. The study draws on institutionalization theory and theories related to gender mainstreaming’s transformative potential. Interviews with politicians and public officials constitute the main material, and are supplemented with official documents. The findings suggest a medium level of institutionalization, with a slightly higher level in one of the municipalities due to more evolved mechanisms for support and quality assurance. The institutionalization appears to be more transformative than perhaps anticipated based on previous research – it is to a greater extent transformative than technocratic.