The mechanism for improved barrier properties against copper diffusion in electroless deposited Co 0.9W 0.02P 0.08 and Co 0.9P 0.1 thin films compared to physical vapor deposition (PVD) of cobalt is quantitatively explained. Secondary ion mass spectrometry depth profile measurements were performed on the films deposited on copper substrates after subjecting them to thermal anneals at approximately half the melting temperature of cobalt. A steady-state mode was observed in the form of concentration plateaus which originate from a combined contribution of grain-boundaries' saturation and copper solubility in the grains. The difference in plateau heights between the samples is assigned to the varying degrees of grain-boundaries' passivation. For Co 0.9W 0.02P 0.08, the copper concentration in the grain boundaries is negligible and the solubility in the temperature region between 550 and 700°C may be described as C S≃6×10 2×exp(-0.52eV/kT) at.%. The higher-copper concentration plateaus in the Co 0.9P 0.1 films are a result of a low, non-negligible copper concentration at the grain boundaries, estimated at less than 10 at.%. The copper concentration plateaus in PVD cobalt were significantly larger due to saturation of the grain boundaries.