The glass transition temperature of nylon 6–inorganic salt mixtures has been investigated. Tg not only increases monotonically with the salt content but the rate of increase depends on the type of salt. Infrared measurements indicate that metal ions form adducts with the amide group, and, as a result, the nylon chains are stiffened. This effect appears to be a major cause of the Tg increase. The adduct formation between the ions and the amide group is very similar to hydration of ions and the observed effect of ions on the Tg of nylon 6 is similar to the effect of the same ions on hydration. The effect is also shown well correlated to Q/R of the ion, the ratio of the oxidation number to ionic radius of the ion.