Thorium (IV) was separated from several anions and cations commonly associated with it in environmental samples by liquid-liquid extraction with N-p-methoxyphenyl-2-furylacrylohydroxamic acid (MFHA) into chloroform at pH 3.6–5.5. The metal was then back-extracted into aqueous media containing 1.8–2.2 M HCl and determined spectrophotometricall) at 500 nm (ϵ = 3.9 × 1041 mol−1cm−1) with 4-(2-pyridylazo) resorcinol [PAR]. The method was successfully applied to the analysis of thorium in coal fly ash, monazite, uranium-thorium ore, animal tissue, plant tissue, and natural waters. MFHA and PAR were chosen from thirteen hydroxamic acids and nine pyridylazo reagents, as the reagent combination that provided the maximum selectivity and sensitivity.