N-(4‘-chlorobenzyl)-4-(chloroacetyl)pyridinium chloride was prepared from the N-(4‘-chlorobenzyl)-4-acetylpyridinium chloride by chlorination with sulfuryl chloride. This chloro ketone had a half life of 150 min at pH 7.3 and reacted specifically with thiols. The reaction product with pentanethiol crystallized as an enol whose structure was determined by X-ray diffraction. The structure indicated conjugation