The comparative reactivities towards cysteine (CySH) of potassium benzyl selenosulfate, potassiump-nitrobenzyl selenosulfate and their thiosulfate isologues have been examined in neutral and slightly acidic media. The selenosulfates (RSeSO−3) reacted more rapidly and gave better yields of products than the thiosulfates (RSSO−3) at pH 5 and pH 7 and in each seriesp-nitrobenzyl derivatives reacted more completely than the benzyl. The products from both types of compound were the cysteine derivatives RX–SCy and CyS–SO−3together with the symmetrical diselenides/disulfides RXXR (X = S or Se). In each class of compound thep-nitrobenzyl derivatives produced a greater percentage of RXXR, at the expense of RX–SCy. The nature and distribution of products indicated that several competing reactions were proceeding simultaneously. An investigation of mechanistic possibilities by way of model systems suggested that, in the case of the selenosulfates, the initial products were RSe–SCy plus SO2-3which reacted further to form RSe−plus CyS–SO−3. The reaction of RSe−with RSeSO−3and RSeSCy produced RSeSeR. By contrast the reaction of RSSO−3with CySH may yield RS–SCy plus SO2-3together with RS−plus CyS–SO−3directly, both sets of compounds are then capable of further reaction.