The corrosion behaviour of X 65 carbon steel in the presence of acetic acid in N-2- and CO2-saturated systems has been investigated using electrochemical techniques. The presence of acetic acid does not influence the anodic reaction but strongly accelerates the cathodic reaction. The cathodic reaction and consequently the corrosion rate of mild steel in the CO2-saturated system increase with increase in acetic acid concentration and temperature. From the values of the apparent activation energies, the corrosion reaction in the absence of acetic acid was found to be under mixed interfacial reaction/diffusion control while interfacial reaction control dominates in the presence of acetic acid. The reduction of adsorbed undissociated acetic acid on the metal surface is proposed as the key species primarily responsible for accelerated corrosion rate at all temperatures.