The interactions between the amino acids (glycine and L-threonine) with some rare earth metal ions (Pr3+, Nd3+, Eu3+, Gd3+, Dy3+, Ho3+ and Yb3+) were studied at a wide range from ionic strengths (0.07-0.32M KNO3) and temperatures (25-45 degreesC) in aqueous solutions by using Bjerrum potentiometric method. The stoichiometric and thermodynamic stability constants were calculated as well as the standard thermodynamic parameters (DeltaGdegrees, DeltaHdegrees and DeltaSdegrees) for all possible reactions that occur. The degree of formation ((n) over bar) for all studied systems was determined and discussed. The thermodynamic parameters differences (DeltaDeltaGdegrees, DeltaDeltaHdegrees and DeltaDeltaSdegrees) were calculated and discussed to determine the factors which control these complexation processes from the thermodynamic point of view. (C) 2003 Elsevier B.V. All rights reserved.