The stepwise stability constants of UO22+, Th4*, Pr3+ and Yb3* ions with 3(acetophenonehydrazone)-6-phenyl pyridazine (AHP), 3-(p-chloroacetopheon- hydrazone)-6-phenyl pyridazine (C1AHP) and 3-p-aminoacetophenonehydrazone)-6- phenyl pyridazine (NH2AHP) have been investigated potentiometrically in 75% (v/v) dioxane-water and 0.1 MKNO3 at different temperatures (298,308 and 318K). For the same ligand, the order of stability decreases in the sequence; Th4* > UOf > Yb3* > P/*. The thermodynamic functions (21(7, AH and zlS) for the dissociation and the complexation reactions have been evaluated and discussed. The higher negative values of AH suggest an octahedral environment around the metal ions in solution. Also, the effect of the dielectric constant of the solvents, mole fraction and ionic strength of the medium on the stability of UO22+-complexes were discussed.