Conductimetric and potentiometric measurements in aqueous solution as a function of temperature show that cupric ions promote the deprotonation of the hydroxy side chain ofthreoanderythroβ-phenylserines. Thethreoligand yields at 298 K the formation constants: Cu(L),K1 = 7.99; Cu(L)2,K2 = 6.71; Cu(L2H−1),K3 = −9.51; Cu(L2H−2),K4 = −10.83. Theerythroisomer yields the corresponding constants:K1 = 7.70,K2 = 6.70,K3 = −9.82, andK4 = −11.20. Thermodynamic parameters for the first two complexation steps are −ΔH = 21.3 and 44.7 kJ mol−1; ΔS = 82 and 132 J K−1mol−1, respectively, for thethreoform and −ΔH = 20.7 and 44.1, ΔS = 79 and 129 for theerythroform. Deprotonation of the complexes corresponds to an additional enthalpy of −21 kJ mol−1. These results and the structures of the complexes are discussed.