A kinetic method for the determination of organosulfur compounds by UV spectrophotometry is described. Organosulfur compounds have been shown to inhibit the Hg(II)-catalyzed substitution of cyanide in hexacyanoferrate(II) by 2-methylpyrazine (2-Mepz). The inhibitory effect is proportional to the concentration of inhibitor and can be used as the basis for the determination of trace amounts of organosulfur compounds such as cysteine, 2,3-dimercaptopropanol (DMP) and thioglycolic acid (TGA). Both the influence of the reaction variables and interference of a variety of ions have been studied. A mechanism for the inhibition process is proposed. The determination range depends on the amount of Hg(II) added and stability of the Hg(II)-ligand complex. Kinetic parameters were determined from Lineweaver-Burk plots, obtained in the absence and presence of the inhibitor. Excellent linearity is observed for all analytes over their respective concentration ranges with correlation coefficient [0.9. The condition calibration curves were linear in the range of 5 9 10 -6 -15 9 10 -6 M for cysteine, 1 9 10 -7 -7 9 10 -7 M for DMP and 1 9 10 -6 -10 9 10 -6 M for TGA. The detection limits were 1.18 9 10 -7 M for cysteine, 4.16 9 10 -8 M for DMP and 1.30 9 10 -7 M for TGA. The effects of amino acids that can interfere in the determination of cysteine were studied.