Depending on the initial concentration of cobaloxime, ethylene production saturates for different reasons. At high cobaloxime concentrations (1.0 mM), catalysis stops due to depletion of BIH, as evidenced by re-addition experiments, in which we first irradiated the system for 20 hours, then selectively re-added various components of the reaction mixture, and irradiated for an additional four hours. These experiments show that it is possible to restart catalysis only upon addition of BIH and no other component of the reaction mixture (Figure ). At low cobaloxime concentrations (1.0 µM), however, where the available supply of BIH is not exhausted over the course of the reaction, catalysis most likely ceases due to degradation of the catalyst. Re-addition of [Co(dmgH)2pyCl] restarts catalysis most effectively compared to re-addition of other reaction components (Figure ). The delicate nature of cobaloxime catalysts has been reported