A theoretical study on first protonation step of a series of triazacycloalkanes with general formula {([X]aneN 3 , X=9–12)} (X=9, L222; X=10, L223; X=11, L233; X=12, L333) is reported. The geometry of all ligands and their monoprotonated forms were fully optimized at both the Hartree–Fock and DFT (B3LYP) levels of theory using 6-31+G∗ basis set. Then the first proton macroaffinities were calculated from the proton microaffinities according to defined equations. It is shown that there are good correlations between the calculated gas-phase first proton macroaffinities of these ligands with their protonation constants in solution.