A method based on controlled potential coulometry has been developed for the determination of thorium, using mercury as a working electrode. A known amount of EDTA is titrated coulometrically in the absence and in the presence of thorium. The difference in Coulombs obtained in these two titrations corresponds to the amount of thorium present in the aliquot. 0.5M sodium acetate has been used as a supporting electrolyte. Precision for thorium determination in the range of 4–8 mg is found to be better than ±0.2%.