The conditions under which organic vapours cause corrosion of bare metals are discussed. The corrosive effects of a series of air-drying paints at 100 per cent RH are evaluated. The drying-oil paints tested were shown to be corrosive as also were the samples of solvent acrylic and nitrocellulose paints. The epoxide, polyurethane and chlorinated rubber paints tested together with a solvent acrylic laquer did not give rise to vapour corrosion effects. Analysis of the vapours evolved by the drying-oil paints have shown them to be mainly formic acid together with saturated fatty acids and aliphatic aldehydes which conformed closely to the theoretically predictable products of the oxidative breakdown of the unsaturated acid residues from which they were derived. The rate of release of volatiles other than formic acid decreased rapidly to very low levels. The rate of formic acid release is shown to decrease to a relatively constant rate which is capable of giving rise to vapour corrosion effects for at least 15 months after painting. A forced drying had little effect on the final steady rate of release of formic acid although it decreased the initial rate. The corrosion caused by the acrylic and nitrocellulose paints is attributed to acetic acid derived from the esters in the paint solvent.