Thorium(IV) reacts with 1-(2′-thiazolylazo)-2-naphthol (TAN) in the presence of antipyrine to form a sparingly soluble red-coloured chelate, soluble in ≥36% methanol (v/v). Complexation takes place instantaneously at pH 2.4–2.8, maintained by glycine buffer. Antipyrine is found to enhance sensitivity of the complex, which is stable for 19 hours. The 1∶2 complex exhibits maximum absorbance at 555 nm, obyes Beer's law in the concentration range from 0.32 to 6.56 μg of thorium(IV) per ml, has a molar absorptivity of 3.14·104 dm3/mol−1·cm−1 and a Sandel sensitivity of 7.4 ng·cm−2. The formation constant (log K) is found to be 8.62 and 8.45. Interference of 57 anions and cations in the determination of thorium(IV) has been studied. From ten repeated determinations, the coefficient of variation was found to be ±0.98%. The method was successfully applied for determination of thorium content in a sample of monazite.