The conformational properties of neutral and protonated N,N＇-dimethyl and N-methyl,N＇-acetyl-guanidines have been studied in vacuo and in solution, considering either water or chloroform as solvents. Using the ab initio MP2/6-31G* optimized geometries obtained in vacuo, single-point HF/6-311 ++ G**//MP2/6-31G* and MP2/6-311++G**//MP2/6-31G* calculations have been performed in vacuo and continuum solvent free energy calculations in solution for most of the syn (S) and anti (A) conformers (with respect to the unsubstituted nitrogen) for different tautomeric structures. The solvation free energy is considerably less favorable, as expected, for neutral than for protonated guanidines (about -11 vs -63 kcal/mol in water and about -4 vs -41 kcal/mol in chloroform for dimethyl-guanidines at the HF level; methyl-acetyl-guanidines show a slightly larger spread around those average values). The correlation contribution to the solvent effect is feeble for the protonated derivatives in solution, while it is larger (up to 4 kcal/mol in water) for the neutral conformers. The cooperative effect among the carbonyl lone pairs and the nitrogen in-plane lone pair is responsible for the fairly large solvent stabilization of the conformers with these polar groups facing each other. Several different conformers/tautomers of neutral methyl-acetyl-guanidine are within a 0.5 kcal/mol free energy range in vacuo, whereas in solution both the neutral (with a cis C=N double bond with respect to the acetyl C=O) and protonated SS methyl-acetyl conformers turn out to be slightly stabilized over the others, especially at the HF level.