A theoretical study on complete protonation of a series of tetrabasic molecules with general formula N[(CH2)(n)NH2][(CH2)(m)NH2][(CH2)(p)NH2] (tren, pee, ppe, tpt, epb and ppb) is reported. For first time, three kinds of gas-phase proton affinities for each polybasic molecule are defined as: 'proton microaffinity (PA(n,i))', 'proton macroaffinity ((PA) over bar (n))' and 'proton overall affinity ((PA) over bar-(ov))'. The variations of calculated log (PA) over bar (n) in the series of these molecules is very similar to that of their measured log K-n. There is also a good correlation between the calculated gas-phase proton macroaffinities and proton overallaffinities with corresponding equilibrium macroconstants and overall protonation constants in solution. (c) 2006 Elsevier B.V. All rights reserved.