The effect of oxygen on the reaction mechanisms in the Si/Ta/Cu metallization system was studied experimentally and by utilizing the thermodynamically assessed Ta–O binary system. It was presented that an interfacial tantalum oxide was formed between Cu and Ta and that it established an additional barrier layer for Cu diffusion. The formation of additional barrier layer was supported by the following observations: (i) No detectable amount of Cu was found from the Ta layer with the combined transmission electron microscopy and energy dispersive spectroscopy at temperatures as high as 650 °C. (ii) Secondary ion mass spectrometry measurements indicated that significant amount of oxygen was incorporated into the films already after the sputtering stage. (iii) 181Ta16O molecular ion signals were detected from the Ta/Cu interface, indicating that the additional layer was in fact some form of tantalum oxide. The diffusion of Cu through the Ta layer could not proceed until the interfacial oxide had been dissolved by the Ta matrix. Since the oxygen solubility in Ta matrix is high in the temperature range of interest, the interfacial oxide dissolution was kinetically controlled. It is to be noted that the threshold temperature range of the dissolution reaction was found to coincide with that of the β–Ta to the bcc-Ta transition, which was anticipated to enhance the kinetics of the dissolution.