High strength steels are widely used in critical applications in the aerospace and automotive industries, but they are subject to hydrogen embrittlement (HE). This study investigates the HE susceptibility of two high strength steel alloys used in aerospace applications: AISI 4340 (Vac Melt) and 300M. Hydrogen embrittlement susceptibility is determined by conducting incremental step load (ISL) tests using a modified version of the test method described in ASTM F1624. Threshold curves are obtained by performing ISL tests both in air and under imposed cathodic potentials (between -1.2 VSCE and -0.7 VSCE) in 3.5 wt.% NaCl solution. Results obtained for the 300M alloy show that the ductile-brittle transition zone in the threshold curve is located between -0.8 VSCE and -0.7 VSCE cathodic potentials. Moreover, the observed average percent NSF (Notch Fracture Strength) for the 300M alloy at an imposed cathodic potential of -1.2 VSCE was lower than 30%. The results are further correlated with microstructural observations and detailed fractography.