Silicon carbide (SiC) presents many advantageous properties for electronic devices designed to work under extreme conditions such as high-temperature (300–600 °C), high-frequency, and high-power. In addition, the formation of an insulating oxide layer (SiO2) by thermal oxidation is an attractive property for the microelectronics industry. Nevertheless, large densities of interface states (Dit) at the SiO2/SiC interface degrade electrical properties of the resulting structure. Such states are responsible for undesirable effects which hamper the development of SiC-based devices. These electrically active defects are partially assigned to compounds named silicon oxycarbides (SiCxOy), that must be passivated. In this work, SiO2 was thermally grow on n- and p-type 4H-SiC using O2 flux bubbled on hot hydrogen peroxide (H2O2), which is a strong oxidizing agent, and acts in SiC surface converting SiCxOy in SiO2. Resulting structures were electrically characterized, and differences in Dit among n- and p-type 4H-SiC substrates were observed. To verify the H2O2 effects on SiC surface composition, oxidations of SiC using 18O as a marker were performed, followed by thermal treatment with H2O2. Samples underwent probing by nuclear reaction analysis and X-ray photoelectron spectroscopy. Results shown a reduction in silicon oxycarbides induced by H2O2. Electrical and physico-chemical data were related in order to explain differences among n- and p-type samples.