This paper describes a nitrogen-based passivation technique for interface states near the conduction band edge in 4H-SiC/SiO2. These states were first proposed by Schorner, et al. , and their origin remains a point of discussion. However, there is now general agreement that these states are largely responsible for the lower channel mobilities that are reported for n- channel, inversion mode 4H-SiC MOSFETs. A post-oxidation anneal in nitric oxide at atmospheric pressure, 1175°C and 200-400sccm for 2hr reduces the interface state density at Ec- E≅ 0.1eV by more than one order of magnitude to approximately 2 × 1012cm2eV-1 The effective channel mobilitiy for lateral n-channel 4H-MOSFETs increases correspondingly from single digits to approximately 30-40cm2/V-s. The mobility for passivated devices exhibits a very weak temperature dependence compared to unpassivated devices for which the mobility increases in proportion to temperature to the power 1.9. 1 he NO passivation process does not significantly affect the breakdown characteristics of thermal oxides on n- and p-4H-SiC. and the beneficial effects of passivation survive post-passivation processing procedures such as the high temperature anneals that are required to form source/drain ohmic contacts.