This paper compares the behavior of the gate oxide in 4H-SiC lateral MOSFETs subjected to post-deposition annealing (PDA) in N2O and POCl3. A significantly higher channel mobility was measured in 4H-SiC MOSFETs subjected to PDA in POCl3 (108 cm2 V−1 s−1) with respect to N2O (19 cm2 V−1 s−1), accompanying a reduction of the interface traps density. Hence, a different temperature coefficient of the mobility and of the threshold voltage was observed in the two cases. According to structural analysis, the gate oxide subjected to PDA in POCl3 showed a different surface morphology than that treated in N2O, as a consequence of the strong incorporation of phosphorous inside the SiO2 matrix during annealing. This latter explained the instability of the electrical behavior of MOS capacitors annealed in POCl3.