This paper studies comparatively the effect of annealing in O-2, N2O, NO and CO2 on interfacial electrical properties of 4H-SiC/SiO2 using MOS capacitors and FETs. From the capacitors, interface trap density (D-it) have been extracted and the NO-annealed sample shows the least D-it while the CO2 annealed sample also shows less D-it than both dry O-2 and N2O samples near the conduction band edge. Extracted from lateral MOSFETs, the Hall mobility is essentially the same (similar to60 cm(2)/V.s) among dry O-2, CO2, and NO samples. The main difference is the inversion electron concentration, which is at least an order of magnitude less in the dry O-2 and CO2 samples. This infers that a high density of interfacial traps is responsible for the lower field-effect mobility. The inversion layer field-effect mobility of CO2 annealed sample is similar to that of dry O-2 (similar to5 cm(2)/V.s) sample and less than that of the NO annealed sample (similar to25 cm(2)/V.s).