The effect of tin on the active corrosion of Fe/18 Cr/20 Ni/2 Mo in non-oxidizing acids was investigated by measuring the weight loss at various potentials and at its rest potential. The corrosion rate in boiling 5 per cent sulphuric acid and 2 per cent hydrochloric acid was remarkably reduced by addition of less than 0.3 per cent tin to the alloy. The potentiostatic weight loss measurement showed that the addition of tin suppressed both the metal dissolution and hydrogen evolution at potentials less than about -40mV, E_h in the sulphuric acid. Regarding the corrosion behavior of the alloy, the stannous ion added into the acid had a similar effect to that of tin in the alloy. The inhibition of metal dissolution was attributed to the presence of a tin-containing film, which is kept adherent on the alloy containing tin due to the reduced disturbance of film formation because of less hydrogen evolution.