The mechanical properties of low strength line pipe steel for energy related industries may degrade by hydrogen induced blister cracking (HIBC) and sulfide stress cracking (SSC) if employed in sour service. Constant load tests in hydrogen sulfide saturated 5% sodium chloride/0.5% acetic acid solution have been conducted on line pipe steels (Grade X-42 to X-70) and show that such steels can, under certain conditions, fail by sulfide stress cracking. In addition, a Grade X-65 line pipe steel failed in less than 24 hours when tested at 100% of the actual yield stress in synthetic seawater saturated with H2S. This same steel, however, showed excellent resistance to HIBC in the BP test (CSR = 0), indicating that superior HIBC resistance may not necessarily provide SSC resistance. The mechanism of SSC in low strength, nickel-free steels is shown to be hydrogen embrittlement by using a constant extension rate test, controlled potential experiments and evaluations of corrosion rate and hydrogen input fugacity. The susceptibility to hydrogen embrittlement was found to be inversely proportional to the hydrogen input fugacity, using the constant extension rate test procedure.