The oxygen behaviour and its influence on the annealing properties of the TiO2/Si and Ti/TiO2/Si systems have been investigated. For the TiO2/Si system no reaction at all could be evidenced after vacuum annealing up to 900°C for 30'. In the Ti/TiO2/Si system metallic Ti reacts with the TiO2 film above 400°C and at 600°C a uniform oxygen solid solution at the solubility limit was obtained without any Si reaction. Silicide formation occurs for annealing temperatures higher than 650°C and causes oxygen expulsion from the reacted layer and consequently a rise in its concentration at the surface where Ti oxide precipitation takes place. This surface oxide layer prevents a further growth of the silicide up to 850°C. The reaction of the whole metal film is attained only by annealing at 900°C or above, when the oxide is completely reduced and an important oxygen loss takes place. A model explaining this behaviour is proposed.