In this study the stoichiometric protonation constants were determined for some alpha-amino acids ( glycine, L- alanine, L- methionine, L- phenylalanine, and L- threonine), the Schiff bases derived from them and 2-hydroxy-1-naphthaldehyde, along with the stoichiometric stability constants of Schiff base - Mn( III) complexes. These equilibrium constants were determined potentiometrically using a combined pH electrode system calibrated in concentration units of the hydrogen ion at 25degreesC, and at an ionic strength of 0.10 mol- dm(-3) NaCl in 30 and 50% dimethyl sulfoxide - water mixtures. The calculations of these constants were carried out using the PKAS and the BEST computer programs. In addition, the effects of solvent composition and structures on the protonation and the complex formation constants were investigated. The mole ratio of Mn( III) to amino acid-Schiff bases was also determined and it was found that the complexes were of the MnL2 type.