Supporting the development and diffusion of sustainable innovations has become a dominant topic on the political agenda of many countries. However, this has proven to be a difficult task. To increase insight in such processes, this paper takes biofuel technologies in the mobility sector as the topic of a comparative case study. Various national governments have supported innovation trajectories around biofuels. We analyse, assess and compare two such trajectories as they have developed so far: one in the Netherlands and one in Sweden. A Technological Innovation System (TIS) approach is applied. A TIS is constituted by actors, networks and institutions, that are to be gradually constructed around a technology. We analyse whether governments and entrepreneurs have succeeded in developing seven key processes, or system functions, necessary for the development and diffusion of biofuel technologies. By analysing the build-up of system functions over time we identify virtuous and vicious forms of cumulative causation. The Dutch and Swedish TISs for biofuels are followed from 1990 to 2005. Our comparison shows that, due to the fulfilment of system functions and the emergence of cumulative causation, the Swedish TIS has reached a market expansion and broad social implementation of biofuels, whereas the Dutch TIS has established considerably less.