Ce (III) and Th (IV) reacts with pyridine-2—aldehyde-2—pyridylhydrazone in aqueous solutions containing 50 % ethanol at pH's 7.75 and 8.45 respectively. Beer's law is obeyed over the range 0.0-15.p.g and 0.0-18 ktg of cerium and thorium respectively. The proposed procedure is sensitive, since molar absortivity is 2.56 x 10 4 and 3.60 X 10 4 1.mol -1 cm-1 for Ce (III) and Th (IV) respectively. The composition and stability of these complexes have been determined. The limitation of this procedure and the effect of other ions have been studied.