The reaction of (2-pyrrole)-(5,6-diphenyl-[1,2,4]-triazin-3-yl)hydrazone with trivalent 4f lanthanide metal ions (LaIII, PrIII, NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII and YbIII) and 5f actinide metal ion UO2II has been studied potentiometrically in 75% (v/v) dioxane-water; at 283, 293, 303 and 313K. The ligand behaves as mono-protic species with the dissociation of hydrazo group proton and reacts with metal ion in molar ratio 1:2; M:L in solution. The formation constants were caulculated, which decrease as temprerature increases. The negative values of ΔH° and ΔG° indicate that the complexation are exothermic and proceeds spontaneously, respectivily. The electrostatic forces are weaker than non-electrostatic forces, where -DG°non is lower than -DG°el.