In this work we investigate the effects of typical thermal treatments on c-Si surface passivation by intrinsic a-SiCx:H films. First, a forming gas anneal (FGA), (400 °C, 30 min in H2/N2) is applied resulting in an improvement in surface passivation. The best result is a surface recombination velocity lower than 23 cm s-1. A second thermal step (730 °C, 30 s) is studied to try and simulate the firing step for screen-printed contacts. This annealing has no effect in surface passivation indicating that a-SiCx:H films are promising candidates for c-Si surface passivation in the photovoltaic industry. In order to analyze the improvement of surface passivation after FGA, a physical model of surface recombination at the a-SiCx:H/c-Si interface is developed. This improvement is directly connected to a reduction in fundamental recombination velocities of electrons and holes at the interface. A possible explanation could be the reduction in the interface state density due to hydrogen saturation of dangling bonds.