Studies for chromatographic separation of Th, U(VI) and Pu(IV) were carried out using mandelic acid as an eluent. The different chromatographic conditions like concentration of mandelic acid, pH of the mobile phase, presence of MeOH and effect of ion interaction reagent (IIR) were studied. The method was optimized for the separation of Th, U(VI) and Pu(IV). At pH<3.5 of mobile phase, Pu(IV) was more retained compared to U(VI) whereas at pH>3.5, reverse trend was observed. The optimized parameters were employed for the separation and determination of Th ,U(VI) and Pu(IV) in a dissolved solution of irradiated (Th,Pu)O₂. Sample treatment was optimized to minimize loss of Pu and Th during chromatographic determination. Studies were carried out using two IIRs to understand the anomalous chromatographic behaviour of Pu(IV) . Retention behaviour of different oxidation states of Pu viz. Pu(III), Pu(IV) and Pu(VI) was also studied in mandelic acid.