Abstract Hydrogen embrittlement of AISI 4340 was studied using slow strain rate tests of smooth dog-bone geometry specimens. Two heat treatments were evaluated that produced yield strengths of 1479 MPa (215 ksi) and 1140 MPa (165 ksi). The strain rate over the gauge section for the smooth tests was 1x10-6 s-1. Testing environments used were air and substitute ocean water (SOW) at room temperature. Tests included specimens that had been precharged with hydrogen in 3.5% NaCl at 50°C for two weeks using a potentiostat to deliver a cathodic potential of -1100 mV vs. SCE to the materials. The slow strain rate tests in substitute ocean water were also performed with an applied cathodic potential of -1100 mV vs. SCE. Results indicated that the AISI 4340 was highly embrittled in SOW under a cathodic potential. The higher strength material showed much more embrittlement. These results will be related to analyses of the fracture surfaces using SEM.