Experimental thermodynamic data for aqueous biomolecules reported in the literature have been combined with group additivity equations of state to generate parameters which can be used to calculate the apparent standard molal Gibbs energies and enthalpies of formation (ΔG° and ΔH°, respectively) and the standard molal third law entropies (S°), heat capacities (C P° ), and volumes (V°) of the 20 common neutral and 5 charged L-α-amino acids as a function of temperature and pressure.‡ Values of C P° and V° for neutral and charged L-α-amino acids minimize, maximize, or exhibit a reverse sigmoid configuration with increasing temperature at P SAT .§ For example, curves depicting C P° of Val, Leu, and Ile minimize with increasing temperature, but those corresponding to C P° and V° of most of the other neutral L-α-amino acids show reverse sigmoid configurations.¶ In contrast, curves representing C P° and V° of the ionized amino acids maximize with increasing temperature. As a consequence, the temperature and pressure dependence of the relative stabilities of the various neutral and charged L-α-amino acids for which C P° and V° exhibit different configurations also differ substantially from one another. Equilibrium calculations indicate that amino acids such as Lys and Arg, which are present almost entirely as Lys + and Arg + at 25°C and pH 7, are ca. 50% dissociated at ca. 125°C and pH 7, where neutrality occurs at ca. pH 6. Such changes in speciation with increasing temperature may have a profound effect on the relative stabilities of other biomolecules such as peptides and proteins at elevated temperatures and pressures.