The complexation of leucine (Leu) with Ce3+ was investigated using the potentiometric method at different ionic strengths (I = (0.10 and 1.00) mol dm(-3)) in KCl medium at elevated temperatures. The stability constants of the leucine complexes with Ce3+ at metal:ligand (M:L) molar ratios of 1:1, 1:3, and 1:5 were determined. From the data obtained, stability constants were calculated using the program HYPERQUAD2006 for proton-ligand and metal-ligand species. The formation of monomeric species was taken into consideration for Ce3+ at (25, 37, and 45)degrees C. Thermodynamic parameters were calculated for all possible leucine complexes. In conclusion, it is indicated that the formation of complexes change the value of the stability constants at various ionic strengths and temperatures.