The thermoelectric power of continuous metal films, calculated with the Boltzmann transport equation for various conductivity models, are compared with Matthiessen’s rule predictions. The three most frequently employed models have been examined : the Fuchs-Sondheimer model, which considers electron surface scattering; the Mayadas-Shatzkes model, which takes into account grain-boundary electron scattering and the total film conductivity model, which accounts for both scattering mechanisms. Theoretical calculations have shown that, whereas deviations appear for external surface scattering, grain-boundary scattering is consistent with the rule. When both scattering mechanisms are modelled concurrently, deviation due to grain-boundary effects appears and can be attributed to the interaction of the two scattering mechanisms.