Effects of H2S and pH on electrochemical corrosion of high-strength low-alloy carbon steel, Grade S-135, were investigated through Tafel slopes. Linear sweep voltammetry (LSV) was conducted in 5 %wt. NaCl solutions of pH 7.9, 10.7, and 12.4 with four H2S partial pressures (pH2S) ranging from 0 to 69 kPa at 85 oC after 60 hours of exposure. Anodic limiting current was observed without H2S, but disappeared at higher pH2S. Cathodic LSV plots did not show limiting currents, suggesting that H2O was the main reactant for the hydrogen evolution reaction at high pH. Both anodic and cathodic currents increased at 69 kPa pH2S compared to 0 kPa. Electrochemical reaction mechanisms were discussed based on anodic and cathodic Tafel slopes. The Tafel slopes from LSV were generally larger than those obtained from electrochemical frequency modulation. Different Tafel slopes could result in a discrepancy in corrosion rate by a factor of 5.