The nature of proton dynamics as well as a pendant side chain's ability for proton dissociation and capture in low-hydration sulfonated polyethersulfone (SPES) (lambda = 2, 4) have been studied theoretically by means of quantum chemical calculations and first-principles molecular dynamics simulations. A detailed comparison of results on SPES with those on Nafion has been made. It is found that the sulfonic groups of Nafion tend to dissociate protons more easily than do those of SPES. Hydration by four water molecules allows the dissociation of a proton from the sulfonic groups in both SPES and Nafion. The results of the first-principles MD simulations on SPES show that the nature of proton transfer kinetics for both hydration levels is very similar. Compared with low-hydration Nafion, however, hydration around the sulfonic groups in SPES is not sufficient to fully dissociate protons from the sulfonic groups, which results from the fact that some of the water molecules participate in hydrating SO(2) groups in SPES rather than SO(3)(-). Such a feature affects the performance of SPES under low-hydration conditions.