We have investigated SiO2/4H-SiC interfaces by using capacitance-voltage (CV) measurements, spectroscopic ellipsometry (SE) and Fourier-transformed infrared (FTIR). From CV measurements, fairly large amounts of interface traps and negatively trapped charges in oxide layers were observed near the SiO2/4H-SiC interface. By using spectroscopic ellipsometry (SE), we found interface layers between 4H-SiC substrates and SiO2 layers, which have higher refractive indices than those of SiO2 and 4H-SiC. The FTIR measurements showed that the Si-O-Si bonds near the SiO2/4H-SiC interface had a lower stretching frequency compared to that of fused quartz. These results suggest that the interface layers are suboxides.