Thorium and rare earth elements (REE) react with 5-(2',4'-dimethylphenylazo)6-hydroxypyrimidine-2,4-dione (I) and 5-(4'-nitro-2',6'-dichlorophenylazo)6-hydroxypyrimidine-2,4-dione (II) in the absence of cetylpyridinium chloride (CPC) to form red complexes. The molar absorptivity and Sandell sensitivity were calculated in absence of CPC. In its presence, REE - complexes are not formed due to miceller masking, whereas Th(4+) has a sensitive reaction with the studied reagents I and II, with enhancement of the color intensity of the complex. Most of the foreign ions are tolerated in considerable amounts; 150-2400-fold amounts of rare earth do not interfere with the determination of thorium. The optimum experimental conditions of the complex formation reactions and the compositions of thorium complexes are described. A simple method is proposed for simultaneous determination of thorium and rare earth element without previous separation.