Studies were carried out on the effect of hydrogen on the mechanical properties of X-70 pipeline steel at different heat treatment conditions. Heat treatments were carried out as-received, under water-quenched, water-quenched and tempered and water-sprayed conditions. The solutions used consisted of 0.1, 0.05, 0.01 and 0.005 M sodium bicarbonate (NaHCO3) for the slow strain rate testing (SSRT) technique and hydrogen permeation measurements at 50 degrees C. Hydrogen permeation increased with solution concentration and was maximum for the quenched steel. Pre-charging of specimens with hydrogen did not have any effect on the mechanical properties of the as-received or water-sprayed steel but it did in the quenched and the quenched+tempered steels. In all the heat treatments, the mechanical properties of the steel were seriously affected only in 0.005 M NaHCO3, the steel in the quenched condition being the most susceptible. The cracking mechanism is dominated by anodic dissolution, film rupture of the steel in the as-received and water-sprayed conditions. But it was dominated by hydrogen embrittlement for the quenched and the quenched+tempered steels. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.