Two sensitive spectrophotometric methods have been proposed for the determination of thorium with phenylfluorone (PF) and quercetin (Quer) in the presence of cetylpyridinium bromide (CPB) and polyvinylpyrrolidone (PVP), respectively. The optimum conditions for complexation reactions have been thoroughly investigated. The 1:3:4 (Th-PF-CPB) ternary complex and the 1:3 (Th-Quer) binary complex in the presence of PVP, are formed in the pH range 8.0-8.6 and 9.8-10.6, with molar absorptivities of 1.55 × 105and 6.25 × 1041.mol−1.cm−1, at 568 and 445 nm, respectively. The methods are rectilinear for 0.10-1.66 and 0.21-4.3 mg Th.l−1and their detection limits (signal-to-noise ratio = 3) are 0.082 and 0.139 mg.l−1, respectively. Results of analysis of thorium in monazite samples using these methods were in good agreement with those obtained by x-ray fluorescence spectrometry.