Hydrogen assisted cracking (HAC) has been investigated in a high strength 4140 steel and a low strength AISI-SAE grade 1022 steel (supplied by Amoco, Naperville, IL-now BP), charged at -50 mA/cm2 in 1N H 2SO4+25 mg/l As2 O3 and tested under threepoint-bend decreasing load. The HAC growth rate for the 1022 steel (1.4×10-7 cm/ s) was found to be approximately two orders of magnitude slower than that of the 4140 steel (3.3×10×5 cm/ s), while the threshold stress intensity factor for the 1022 steel (37.0±1.0 MPa m1/2) was significantly higher than that of the 4140 steel(7.0±0.5 MPa m1/2). This research develops an analytical model, based on the hypothesis that hydrogen both reduces crack resistance (R) and increases crack driving force (G), to explain HAC in 4140 and 1022 steels. The model predicts the hydrogen concentration required to initiate HAC as a function of the applied stress intensity factor andyield strength of the steel. Hydrogen-induced reduction in R was found to dominate HACin the 4140 steel, while hydrogen-induced reduction in R was combined with an increasein G for HAC cracking of the 1022 steel. Copyright © 2009 by ASME.