The acidities of a series of fully hydrated sulfonated poly(styrene-co-divinylbenzene) resins with varying levels of sulfonation from 0.80 to 5.25 mequiv g-1 have been characterized. Enthalpies of neutralization with aqueous NaOH have been measured by titration calorimetry. The degrees of acid dissociation (α) have been measured using FT-Raman spectroscopy, based on the intensity of the –SO3- stretching band at 1033 cm-1. The same measurements have been made on aqueous solutions of p-toluenesulfonic acid (p-TsOH), on the basis that these solutions are analogs of the internal solutions in the hydrated resin gels. For resins with low levels of sulfonation, and therefore relatively dilute internal acid solutions, the internal and the equivalent p-TsOH solutions are similar. However, significant differences are seen in highly sulfonated resins where the internal solution concentration is above 4.0 mol kg-1. At these concentrations, p-TsOH solutions show essentially complete dissociation and enthalpies of neutralization typical of a strong, fully dissociated acid in aqueous solution. In contrast, the acid groups in hydrated resins are largely undissociated and exhibit numerically higher enthalpies of neutralization, and catalytic activity measurements indicate that these acid groups are stronger than normal. It is proposed that this acidity enhancement is associated with networks of sulfonic acid groups which form in highly sulfonated resins. Although such networks have been previously proposed in dehydrated resins, this paper reports the first evidence for their existence in resins under conditions of full hydration.