The protonation constants of(+/-)-norvaline and the stability constants of complexes between lanthanide ions and (+/-)-norvaline at various ionic strengths I-c/mol . dm(-3) = 0.05, 0.10, and 0.15) at 300 K and at different temperatures (T/K = 300, 310, and 320) at I-c = 0.05 mol . dm(-3) were determined potentiometrically. The potassium nitrate solution was used to maintain the ionic strength. The stability constants show an inverse relationship with ionic strengths. The thermodynamic parameters based on these formation constants were calculated. The values of enthalpy change and entropy change are positive for all systems. The stability order obtained was La(III) < Pr(III) < Nd(III) < Tb(III). The formula of the complex is given and acid-ligand curves are interpreted. The thermodynamic stability constants (log K-n(T)) obtained for the complexes of(+/-)-norvaline with La(III), Pr(III), Nd(III), and Tb(III) were 5.57, 5.94, 5.96, and 6.55, respectively.