The complex formation equilibria between lanthanoid(III) and DCPT ions have been studied by potentiometry in 75% (v/v) dioxane-water mixture and 0.10 M KNO3 as constant ionic medium, at 10, 20, 30 and 40 degrees C. A series of mononuclear protic [Ln(HL)](2+), [Ln(HL)(2)](+), [Ln(HL)(3)], [LnL(HL)(2)](-), [LnL(2)(HL)](2-) and non-protic [LnL(3)](3-) complex species were found in solutions and their formation constants, ethalpies and entropies were determined. It was established that the stepwise Gibbs energies of the protonated complexes varied in the sequence - Delta G(Ln(HL)) > - Delta G(Ln(HL)2) > - Delta G(Ln(HL)3), and the stepwise enthalpies the order was - Delta H-Ln(HL > - Delta H(Ln(HL)2) > - Delta H(Ln(HL)3), and the entropy markedly increased in the order Delta S-Ln(HL) > Delta S(Ln(HL)2) > Delta S(Ln(HL)3). The results are discussed in relation to changes in the metal-water and metal-ligand bond strength. The variation of the thermodynamic parameters as a function of ionic potential of lanthanide elements is discussed in terms of the different hydration numbers of lighter and heavier lanthanide(III) ions.