Green public procurement is believed to have the potential to contribute to environmental improvement and diffusion of green technologies. The aim of this paper is to compare and analyse how two Swedish regions use public procurement to promote the introduction of renewable fuels in their public bus transport systems. The method is a qualitative comparative case study, based on interviews and document studies. The paper addresses the questions of what the strategic motivations are for using public procurement to stimulate renewable fuels, and what the practical challenges have been in relation to five important factors identified from previous research: strategies, requirements, costs, size and knowledge. In one region, procurement is used in a strategic way to create a local market for biofuels, which poses higher demands on political backing, information and knowledge, the way requirements are set, and an acceptance of increased costs. In the other region, procurement is used instrumentally to increase the share of biofuels in a cost-effective way that gives room for more flexibility and reduces the demands on the procurers. This paper highlights the importance of context when assessing green public procurement schemes and analyses the case-specific influence of factors on the outcome of green public procurement.