Photolysis of alkylmercury halides in the presence of electronegatively-substituted 1-alkenes yields adduct radicals [RCH(2)CH(EWG)(.)] that in some cases react with RHgX to form RCH(2)CH(HgX)(EWG), e.g., EWG = (EtO)(2)PO or PhSO(2). When the EWG is carbonyl or cyano, the resonance stabilized adduct radicals fail to react with the alkyl mercury halide. In these cases photolysis with RHgCl/KI in Me(2)SO leads to the adduct mercurial via reaction of the adduct radicals with RHgI(2)(-). The reactions of tertiary-enolyl adduct radicals are inefficient with RHgX/KI, and disproportionation of the adduct radicals is the major reaction pathway. For secondary- or tertiary-adduct radicals the reductive alkylation products are formed in excellent yield by reaction with RHgCl and silyl hydrides in Me(2)SO solution in a process postulated to involve RHgH as an intermediate. The relative reactivities of a number of alpha,beta-unsaturated systems toward t-Bu(.) have been measured by competitive techniques. The results demonstrate a high reactivity of s-cis enones relative to the s-trans conformers.