The stability constant on complexation of VO2+ with leucine in mixed solvent systems of methanol + water and dioxane + water have been determined spectrophotometrically at (25 +/- 0.1) degrees C and I = 0.15 mol.dm(-3) of sodium perchlorate as ionic medium in the pH range (1.20-10.20) with high ligand-to-metal ratios. Linear relationships are observed when log K is plotted versus 1/D, where K and D represent stability and dielectric constants, respectively. The results are discussed in terms of the effect of solvent on protonation and complexation.