The acid-base chemistry of multidentate (bio)ligands are usually characterized in terms of macroscopic protonation constants, log(10) K (or pK), which are composites of the microscopic constants (log(10) k or pk) for the individual groups. In this study, macroscopic protonation constants of some alpha-amino acid (glycine, L-alanine, L-valine, L-leucine, L-phenylalanine, L-serine) and protonation constants of their methyl-and ethyl-esters in dimethyl sulfoxide-water mixtures (10-40 % dimethylsulfoxide by volume) were studied by potentiometric technique at 25.0 +/- 0.1 degrees C under a nitrogen atmosphere, and at ionic strength of 0.10 mol.L (1) sodium chloride. The protonation constants were calculated by using the Best computer program and the constants were evaluated for microscopic protonation constants of these alpha-amino acids. The protonation constants are discussed on the basis of solute-solvent interactions. The constants were influenced by changes in solvent composition. The dipolar ionic form, however, still greatly predominates, even in 40 % dimethyl sulfoxide.