Abstract: Investigation was made into the operating conditions for the preliminaryseparation of uranium and plutonium from the dissolution solution of irradiatedfuels by TOPO--cyclohexane, and the extraction of total rare earths by PMBP-benzene. The content of total rare earths is determinated spectrophotometricallywith chlorophosphonazo--mA as the color developer. The optinium conditionfor reducing the absorptivity of yttrium with (NH_4)_2C_2O_4 was proposed. Underthis condition the absorptivities of yttrium and the cerium group tend tobe similar. The recovery of each rare earth element is about 97%. By means ofthis procedure the content of total rare earths in the irradiated uranium fuels isdetermined and the burn--up of the fuels is calculated. The result is consistentwith that obtained by mass spectrometric and radiochemical methods with adeviation less than ±2%. The procedure may be applied to the determinationof the burn--up of irradiated uranium fuel.