This paper introduces a method for the determination of the gate oxide thickness, Xox, of N- and P-Channel MOSFETs with ultrathin oxides based on the characterization and modeling of the substrate current resulting from valence-band electron tunneling (VBET) in the direct-tunneling (DT) regime. Under certain bias conditions, valence-band electron tunneling becomes the main constituent of the substrate currents in N- and P-MOSFETs. This method has several advantages over other methods for the determination of Xox, and yields values of Xox that agree well with those obtained from modeling capacitance-voltage characteristics, C(V), while taking quantum-mechanical effects into account. Its main advantage is that it is not limited by the oxide thickness.