A study of the reactive co-evaporation of Si and Ge in O-2 atmospheres to obtain amorphous Ge:Si:O thin films is presented. The film composition has been measured using RES, EDX and NRA. The dependence of the Si to Ge concentration ratio with deposition parameters (single deposition rates, oxygen pressure and substrate temperature) is analysed. The deviation between the Si to Ge ratio in the films and the ratio of the values set in thickness controllers does not depend on the substrate temperature but is influenced by the presence of an oxygen pressure. A simple model to explain the process behaviour is presented.