Molecular model systems based on propyl phosphonic acid (ppa) were studied by means of density functional theory calculations in order to describe the acid-acid interaction and the formation of the hydration sphere. The formation of ppa dimers is reported and the energetic difference between two dimer structures is presented. The hydration sphere of ppa was represented by model systems ppa(H2O)(n), for which the system with n = 4 formed the first hydration sphere (h(1)), while n = 7 can be considered a good approximation to the complete inner hydration sphere around the phosphonic acid group. The study of the ppa-H+ (H2O)(n) model systems showed an interesting structural behavior comparatively to the ppa(H2O)(n) systems. The protonated acids exhibited equivalent phosphorous-oxygen bonds and a general molecular structure is proposed to represent these protonated species. (c) 2005 Elsevier Ltd. All rights reserved.