Nitridation of the SiO2-4H-SiC interface using post-oxidation annealing in nitric oxide (NO) is the most established process for obtaining high quality 4H-SiC MOS interfaces. In this paper, a detailed study of the interfacial nitrogen uptake kinetics is reported for the (0001) Si-terminated and (000-1) C-terminated crystal faces of 4H-SiC. The results indicate a significantly faster kinetics for the C-face compared to the Si-face. For the first time, the correlation between N-uptake and the interface trap density reduction was observed on the C-face. First-order kinetics models are used to fit the nitrogen up-take. Empirical equations predicting N coverage at the SiO2-4H-SiC interface via NO annealing have been established. The result also shows that the N-uptake rate is associated with the oxidation rate. (C) 2014 Elsevier B.V. All rights reserved.