Polysilicon gated MOS capacitors with oxide thicknesses between 22 nm and 57 MI were fabricated on n-type 4H- and 6H-SiC epitaxial layers with net doping concentrations in the lower 10(16) cm(-3) range. The MOS capacitors were characterized by Voltage ramping and constant current stress with simultaneous monitoring of the trapped oxide charge. Breakdown field strengths up to 12 MV/cm and charge-to-breakdown values up to 30 C/cm(2) were achieved for the thinnest oxides on 6H-SiC. For the thicker oxides, the charge-to-breakdown is more than 2 orders of magnitude lower. This effect was correlated to impact ionisation in the oxide. The charge monitoring showed that positive charge trapping in oxides on SiC is more pronounced than in oxides on silicon. Furthermore, the positive charge increases significantly for increasing oxide thickness due to impact ionisation.