Hydrogen assisted cracking (HAC) has been investigated in high strength 4140 and low strength Z17D pressure vessel steels, charged at -50 mA/cm 2 in 1N H2SO4 + 25 mg/1 As2O 3 and tested under three-point bend decreasing load. The HAC growth rate for Z17D steel (1.4×10-7 cm/s) was found to be approximately two orders of magnitude slower than that of 4140 steel (3.3×10-5 cm/s), while the threshold stress intensity factor for Z17D steel (∼37 MPa√m) was significantly higher than that of 4140 steel (∼7 MPa√m). This research will show that a single analytical model, based on the hypothesis that hydrogen both reduces crack resistance (R) and increases crack driving force (G), can explain HAC in 4140 and Z17D steels. The model predicts the hydrogen concentration required to initiate HAC as a function of the stress intensity factor and yield strength of the steel. Hydrogen-induced reduction of R was found to dominate HAC in 4140 steel, while hydrogen-induced reduction of R was combined with an increase in G for HAC cracking of Z17D steel.