The role of polar interface phonon scattering in determining mobility has been investigated in inversion layers of MOS structures on InAs and Si. The comparison between mobility measurements and transport calculations is found to provide an alternative method for determining interface properties. Transport at the Si/SiO2 interface is found to be dominated by several scattering mechanisms at various inversion densities. However, transport at the InAs/oxide interface is dominated entirely by surface roughness, because of the larger interface widths in this system. Interface phonon contributions are small.