The room‐temperature surface mobility of free carriers in metal‐oxide‐semiconductor systems has not been satisfactorily explained by treating phonons as the only important scattering mechanism. The present work shows that, in addition to phonon scattering, both Coulombic and surface‐roughness scattering must be taken into account. Experimental results are presented showing that, contrary to the prediction of phonon scattering, surface mobility is process dependent, and that a strong correlation is observed between the detailed mobility behavior and the surface‐state density and the degree of surface roughness. A theoretical calculation of surface mobility, based on the combined effect of scattering due to phonons, charged centers, and surface roughness, is also presented.