Two spectrophotometric methods are described for the determination of tiopronin in pharmaceuticals. They are based on the oxidation-reduction reaction between tiopronin and iron (III), then forming a complex between iron (II) and ferrozine or di-2-pyridyl ketone-2-thiophenoylhydrazone. The produced colored iron (II)-ferrozine complex [system I] absorbs at 562 nm, while the iron (II)-di-2-pyridyl ketone-2-thiophenoylhydrazone complex [system II] absorbs at 656 nm. The effect of different factors such as: pH, reagent concentration, time of reaction, temperature and the tolerance amount of the common excipients have been studied. Applying the optimum

4 and 1.0x104 l mol-1cm-1  for systems I and II, respectively. Both methods offer high selectivity, sensitivity and accuracy with a relative standard deviation (RSD) of less than 1.1% for five measurements. The proposed methods were applied successfully for the determination of tiopronin in

Captimer tablets.