Process optimized MOSFETs yield mobilities of roughly 25 and 100 cm(2)/Vs for 4H- and 6H-SiC respectively. In order to explain these results, we measure the interface state density near the conduction band for both polytypes using the AC conductance technique. The 4H MOS interfaces suffer from greater bandtailing, with D-IT exceeding 1x10(13) eV(-1)cm(-2) Integrating this DIT profile, we estimate a density of 4x10(12)cm(-2) interface states lying below the Fermi level at the onset of inversion. According to the Si model this density of charge at the interface will cause an order of magnitude reduction in mobility due to Coulombic scattering. Also, half of the induced mobile charge will be trapped by these states, thereby reducing the measured mobility by another factor of two. Applying these reductions to the bulk mobility yields expected mobilities that are consistent with measured data.