Bicontinuous microemulsions made from dodecane, water, and didodecyldimethylammonium bromide (DDAB) were investigated as media for the catalytic reduction of trans-1,2-dibromocyclohexane and for S(N)2 reactions of n-alkyl bromides with electrochemically generated Co(I) complexes. Macrocyclic complexes vitamin B-12 (a cobalt corrin) and Co(salen) resided in the water phase, while the alkyl bromides resided in the oil phase of the microemulsion. Rates of these bimolecular reactions were comparable in bicontinuous microemulsions to those in homogeneous solvents. Rates of DBCH reduction 40-fold larger in the bicontinuous fluid than in a water-in-oil microemulsion may be caused by a larger interfacial area of the bicontinuous system. For a given alkyl halide, a linear relation between log k(1) and E(o)’(Co(II)/Co(I)) was found for both catalytic and S(N)2 reactions for rate constants in DMF and the microemulsion. Thus, kinetic differences are controlled by activation free energies governed mainly by the formal potential of the Co(II)/Co(I) redox couple, rather than by distribution of reactants between phases. Formal potentials in the microemulsion depended on specific interactions, such as those of [Co-I(salen)](-) with cationic surfactant head groups or the influence of water phase pH on vitamin B-12.