A detailed non-LTE study on the abundances of C, N, O, S, and Zn was carried out for five F-G supergiants of suspected post-AGB candidates with IR excesses (HD 112374, HD 148743, HD 161796, HD 163506, and HD 187203), since these volatile elements can provide the key to inferring the original metallicity as well as an evolution-induced change due to mixing, given that they are free from depletion caused by dust-gas separation.