The formation constants of the species formed in the systems H+ + Mo(VI) + aspartic and glutamic acids and H++ aspartic and glutamic acids have been determined in different aqueous solutions of methanol [(0 to 40) % v/v] at 25 degrees C and constant ionic strength (0.1 mol . dm(-3) sodium perchlorate), using a combination of spectrophotometric and potentiometric techniques. The composition of the complexes was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1: 1 complex with the amino acids of the type MoO3L2- at pH 5.8. The protonation and formation constants in various media were analyzed in terms of Kamlet and Taft's parameters. Single-parameter correlations of the formation constant, K-S, versus alpha (hydrogen-bond donor acidity), beta (hydrogen-bond accepter basicity), and pi* (dipolarity/polarizability) are poor in all solutions, but multiparameter correlations represent significant improvements with regard to the single-parameter models. Linear correlation is observed when the experimental log K-S values are plotted versus the calculated ones, while all the Kamlet and Taft's parameters are considered. Finally, the results are discussed in terms of the effect of solvent on protonation and complexation.