A simple and rapid spectrophotometric method for determination of thiacetazone is described. Thiacetazone forms a stable greenish coloured complex with Cobalt (II) under approprjate experimental conditions. Maximum absorbance occurs between 660 and 690 nm. The values are reproducible at 680 nm and Beer's law is obeyed at this wavelength. Thiacetazone is determined in dosage form and in presence of other anti-tubercular drugs. It is also determined in presence of commonly used drugs such as chlorpheniramine maleate, riboflavin, pyridoxine hydrochloride and dicalcium phosphate.