The thermodynamic parameters of complexes of UO2(II) and Th(IV) cations with arylbis(5-hydroxy-3-methyl-1-phenylpyrazol-4-yl)methanes and other derivatives have been measured in 75% (v/v) methanol-water and 0.10 M KNO3 using potentiometry. Comparison of the experimental and calculated (using the modified Born equation) values of DELTAG indicates the inner sphere nature of all the complex systems. The solid complexes were isolated and characterized by elemental analysis, conductance, infrared spectra, and by TG, DTG and DTA measurements. The ligands behave towards the metal ion as monobasic bidentate ligands.