Electron mobility and free carrier density have been measured in 6H-SiC MOS inversion layers using the Hall effect to study unexpectedly low transconductance in SiC MOSFETs. We find that the Hall mobility can be much larger than the effective mobility extracted from transconductance data. In addition, the actual free electron density is found to be smaller than predicted by the charge-sheet model, which is caused by substantial electron trapping at the SiC/SiO2 interface.