Radio frequency (rf) plasma oxidation at the floating potential of tantalum silicides, TaSix(0.3≤x≤4.5), has been investigated. The oxidations were carried out in the 400–800 °C temperature range. Measurements of the overall oxygen (16O and18O) and cation contents as well as their distribution in the films were carried out using Rutherford backscattering spectrometry and nuclear microanalysis techniques. The oxidation rate was found to be primarily controlled by the diffusion of the oxidizing species through the oxide. The influence of silicide composition, temperature, and substrate nature on the nature of the oxide has been analyzed. The rf plasma oxidation of TaSixsilicides can be understood as the competition between two diffusion processes: oxygen diffusion from the plasma through the growing oxide and silicon migration from the silicide layer or from the silicon substrate. Both silicon and oxygen diffusion processes coexist and determine the nature of the grown oxide. The mechanisms of oxygen movement were studied using18O as the tracer. The data show the maintenance of the order of oxygen atoms during oxide growth and a partial exchange of16O present in the oxide with the18O of the plasma. These results show that the microscopic mechanism of oxygen transport involves a short‐range oxygen migration.