Based on a thermodynamic cycle the pKa in aqueous phase of the series of molecules of the type (Me)2-N-Phenyl-(HC=CH)n-CHO with n = 0, 1, 2 and 3 have been determined. To that end the SM5.4 solvation model has been considered. The calculated pKa have been compared with the pKa measured experimentally. A study has also been made of the proton affinity in the gas phase and in aqueous phase. The calculation scheme agrees favorably, in a qualitative manner, considering favorably the molecular and solute-solvent interaction characteristics that determine the free energy that governs the acid-base properties of the molecules in the series.