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Silicon carbide based metal-oxide-semiconductor (MOS) devices are attractive for gas sensing in harsh, high temperature environments. The response of catalytic gate SiC sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. We have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation (high capacitance) to inversion (low capacitance) occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena - a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states at the SiO2/SiC interface. MIS capacitance sensors typically operate in constant capacitance mode. These results. affect sensor sensitivity since the slope of the C-V curve changes dramatically with gas exposure.