The applicability of the resonance model to explain the evolution of electron density was tested for a set of 15 nitriles whose protonation processes were studied by means of the quantum theory of atoms in molecules (QTAIM). The electron densities were obtained at the B3LYP/6-31++G**//B3LYP/6-31++G** and HF/6-31++G**//HF/6-31++G** levels. QTAIM atomic and bond properties do not follow the trends that should be expected according to the resonance model and our results are more in line with a H+-N C-R Lewis structure than with the H-N+ C-R and H-N=C+-R ones. Also, reasonable agreement between experimental and calculated PA values as well as good correlations between variations in atomic energies and populations as a result of protonation were found.