Catalytic electrodes were prepared by covalently binding poly-L-lysine (PLL) onto oxidized carbon electrodes and then linking the cobalt corrin vitamin B-12 hexacarboxylic acid [B-12(COOH)(6)] to this surface. Additional layers of PLL-B-12(COOH)(6) were attached in a similar way. Quartz crystal microbalance studies showed regular and reproducible layer formation. Electrochemical and catalytic properties of the (CoL)-L-II/ (CoL)-L-I redox couple in these films were investigated using voltammetry and preparative electrolysis in an sodium dodecyl sulfate microemulsion. These films obeyed theoretical predictions of a maximum in voltammetric catalytic efficiency as thickness increased for the reduction of 1,2-dibromocyclohexane (DBCH) in a microemulsion. In films with less' than optimum thickness, kinetic control of the chemical reaction between (CoL)-L-I and DBCH predominated. As film thickness was increased beyond that found for maximum efficiency, electron and reactant mass transport within the films became limiting factors. Under synthetic electrolysis conditions, optimal turnover numbers were found for very thin films on porous electrodes, and best yields and current efficiencies were obtained with the relatively small catalyst coverage of about 2 nmol cm(-2).