Co-feeding experiments have shown that propene can re-participate in the synthesis reaction by simultaneous bond fissions and formation. The resulting catalytic behaviour depends on the propene concentration. At low concentrations (1 mol% propene), bond fission is very pronounced and results in a substantial increase in the rate of methane formation. At higher concentrations (5 mol% propene), no bond fission is observed and synthesis participation results in a slightly higher synthesis activity. At still higher concentration (10 mol% propene), synthesis activity is suppressed by site occupancy by propene. Propene acts as a scavenger of surface hydrogen, thereby causing a propene concentration-dependent increase in the olefin selectivity and a decrease in the methane selectivity.