The electrochemical oxidation of catechol in the presence of TSC in buffer solution (pH 5.3) was studied by cyclic voltammetry and controlled potential coulometry. The controlled potential coulometry was carried out with composite graphite working electrode. The oxidation mechanism was then suggested. The electro-generated o-quinone had a 1, 4-addition reaction with the available thiosemicarbazide, such that an increase in the current, which was attributed to the re-oxidation of the thiol-catechol adduct, could be used as a mean of quantifying the thiol. The electrochemical reaction which produced quinoid intermediates and their subsequent reaction with sulphydryl compounds have been investigated and were later quantified. The concentration of thiosemicarbazide in aqueous solutions was determined according to the linear dependence of the anodic peak current with concentration. The calibration plot was linear in the ranges of 1 x 10-5 to 8.5 x 10-4 mol L-1 with linear regression coefficient (R2) of 0.995. The relative standard deviation of the determination was 2% (n = 6).