The course of inactivation of yeast alcohol dehydrogenase (YADH) using 4,4'-dithiodipyridine (DSDP) has been studied in this paper. The results show that the reaction mechanism between DSDP and YADH is a competitive, complexing inhibition. The microscopic constants for the inactivation of the free enzyme and the enzyme-substrate complex were determined. The presence of the substrate NAD+ offers strong protection for this enzyme against inactivation by DSDP. The above results suggest that two Cys residues are essential for activity and are situated at the active site. These essential Cys residues should be Cys-46 and Cys-174 which are ligands to the catalytic zinc ion. Another Cys residue, which can be modified by DSDP, is non-essential for activity of the enzyme.