Characteristics of mobile ions in the SiO2 layers of 6H-SiC MOS structures have been studied by capacitance-voltage and thermally stimulated current measurements. Thermal oxidation was used to grow oxide layers on the Si-face of 6H-SiC, and post-oxidation annealing was performed in Ar-ambient. A single thermally stimulated current (TSC) peak was observed at about 450 K in the measured temperature ranges, and the peak is generated by positive mobile ions. The peak increased slightly and clearly shifted to higher temperatures with increasing the bias voltage V-b up to 3.0 V The mobile charge Q(TSC) obtained from the TSC curves increased as V-b increased, and almost coincided with Q(CV) obtained from the flatband voltage shift in C-V characteristics. It is found that the behavior of mobile ions in SiO2 layers of SiC-MOS capacitors is similar to that of Si-MOS capacitors.