Electrochemical behaviour of mild steel in pure glacial acetic acid and contaminated with formic acid at different temperatures has been investigated and discussed in the present communication. Anodic and cathodic polarization behaviour have been studied using potentiostatic technique. Anodic polarization behaviour of mild steel in glacial acetic acid and in presence of different amounts of formic acid showed active behaviour in the whole range of the applied potential. On the addition of 0.5 M sodium salts to the acetic acid, active-passive-transpassive behaviour for most of the mixture compositions was observed. The cathodic polarization behaviour observed both in the absence and presence of the salt shows a shift of the curves towards higher current density with increasing concentration of formic acid.