Thermal stability of diamond-like carbon is still a problem which limits the potential applications of this material. With the aim of obtaining a material with increased stability, the behaviour of silicon incorporated hydrogenated amorphous carbon films (a-C1-xSix:H) under thermal annealing was investigated. Results show that the observed effects can be divided in two groups : low temperature (T < 400 degrees C) and high temperature (T > 400 degrees C) effects. In contrast to what is observed in case of pure a-C:H, several of the properties of the films were found to change in the low temperature range. A detailed analysis of the infrared absorption spectra showed that the density of sp(3) C-H bonds is increased while the olefinic sp(2) ones are reduced by annealing. A shift of the Si-H stretching mode to lower wavenumbers is also observed, indicating that void elimination may occur due to bond reconstruction. In this way, a material with reduced spin density, smaller residual stress and increased optical gap results, indicating an increased polymeric character. Upon annealing in the high temperature range, the material degradation processes of hydrogen loss and graphitization start to occur. In the case of films with low silicon content : the hydrogen effusion process is associated to the graphitization of the material. As the silicon content is increased, hydrogen effusion is shifted to higher temperatures and graphitization is inhibited due to the increased disorder presented by these films.