Abstract The gas phase proton transfer for the system H 2O ⋯ HX (XF, Cl and OH) is analyzed in detail using density functional theory. The process is decomposed into H-bonding, ion pairing and dissociation contributions by means of a thermodynamic cycle that uses a water molecule as a proton relay agent. For each step, the energy variations are partitioned into its electrostatic and non-electrostatic contributions. The analysis shows that the principle of maximum hardness is fulfilled for all three processes considered in the proton transfer reaction. The present approach surprisingly yields accurate values for the proton affinity of the F −, Cl − and OH − ions.