The electrical properties of Al2O3 as a gate dielectric in MOS structures based on n- and p-type 4H-SiC grown by sublimation method have been investigated and compared to the properties of similar structures utilizing SiO2. The electrically active defects in the structures are studied by CV method. The results show that the type as well as spatial and energy distribution of defects in Al2O3/SiC and SiO2/SiC samples are different. The structures with Al2O3 on p-type 4H-SiC demonstrate much better C-V characteristics than the p-type 4H-SiC/SiO2 structures.