The kinetics of proton transfer of acids, bases, and water in aqueous solutions has been studied at the ab initio level, using a nonlocal exchange and correlation density functional and an extended basis set. As a first step toward a full comprehension of the process, we have used as model system the X ...(H2O)(4) clusters (X = NH3, H2O, H2S, and HCl). These are the smallest water clusters for which the neutral form, B ...(H2O)(3) ... AH, and the double ionic form resulting from the proton transfer, BH+...(H2O)(3) ... A(-), are minimum-energy structures in the potential energy surface of the X ...(H2O)(4) cluster. Our results show that for these clusters the transfer takes place following a concerted mechanism, A transition state is found for all clusters except for the H2O ...(H2O)(3) ... HCl one, which shows a strong tendency to dissociate into the more stable double ionic form without barrier. For the other clusters, there is a small transition state displaced toward the double ionic products, due to the higher stability of the neutral cluster. Therefore, these clusters present a weak acid or base behavior, except the H2O ...(H2O)(3) ... HCl cluster which follows one of a strong acid.