The conformational properties in vacuo and in solution of neutral and protonated N,N’-diphenylguanidines have been studied making use of ab initio SCF and MP2 calculations in vacuo and continuum solvent free energy calculations in solution on the STO-3G and 4-31G optimized geometries obtained in vacuo. For the N,N’-diarylguanidine series with condensed rings (di-1-naphthyl, di-1-anthracenyl, and di-9-anthracenyl derivatives) the calculations have been carried out on the relevant partial charges located at the molecular mechanical geometries derived from the AMBER force field in MacroModel. The STO-3G solvation free energy is considerably less favorable than the 4-31G one for the diphenylguanidines, but only about 2 kcal/mol less favorable for the diphenylguanidinium rotamers, which an the whole show a better agreement between the two basis sets. The STO-3G basis set predicts the syn-anti (SA) structure to be the most stable in solution for neutral diphenylguanidines, whereas the 4-31G basis set favors the AS rotamer. Extended basis set calculations on N-phenylguanidine favor the anti structure as well. The MP2/4-3 1G//4-3 1G correlation corrections applied in solution produce a solvent effect lower than at the SCF level, and only the salvation free energies of AA and AA+ are reduced with respect to that of AS. The cavitation free energy is nearly independent of the basis set. The salvation free energy obtained from the partial charge description of the diphenyl derivatives is analogous to that produced by the STO-3G basis set for the neutral rotamers, while it is about 10 kcal/mol more favorable than the corresponding STO-3G values for the protonated conformers. The free energy ab initio and electrostatic results are in fair qualitative agreement with those obtained with the GB/SA method. The stacking of the aromatic rings is followed by unfavorable electrostatic and favorable van der Waals interactions. The hydration of these rings is slightly favored by the GB/SA method in a nonstacked and rather separated form.