The gas phase acidities GPA for deprotonation) of eighteen of the naturally occurring amino acids are evaluated. The solution energy differences, ƒxGsol(HA) -ƒxGsol(A(-) are calculated trom aqueous acidities. Three sets of values are obtained based on the side chains.for the different side chains support the avalilabe GPA. These are supported by semi-empirical multiconfiguration configuration interaction calculations of bond dissociation energies and electron affinities of the deprotonated radical. The GPA of aspartic and glutamic acid which have not been measured are predicted from empirical and theoretical considerations. Potential experimental and theoretical methods of supporting the GPA and hydration energies are suggested based on similar studies of the nucleic acids.