We have investigated the growth of Co2Si and CoSi around 400°C in samples of Si/Co and Si/CoSi/Co. We selected the Co-Si system because CoSi is known to have a larger heat of formation than Co2Si, hence the former should be favorable for formation from the viewpoint of free energy change or driving force. However, we found that Co2Si is the one which always grows first. This leads us to conclude that it is not the driving force but rather the kinetics which governs the selective growth of thin-film intermetallic compounds. Thus, we propose here that the first phase formation is selected by the one with the lowest kinetic barrier.