Co-feeding experiments have shown that both branched and linear butenes can initiate chain growth, although they are unlikely to propagate hydrocarbon synthesis. At low co-feed concentrations, up to ca. 5 mol%, a slight increase in overall activity can occur and the rate of methane formation is moderately decreased. At higher co-feed concentrations of ca. 10 mol%, synthesis activity is suppressed and methane formation is inhibited more severely. The latter is attributed to surface-hydrogen scavenging and site occupancy by butene. No indication of surface-intermediate scavenging is found. The probability of chain growth initiated by butene is higher than the probability of chain growth initiated by synthesis gas. When alkenes are co-fed, the olefin selectivity increases in the order of constant concentrations of ethene < propene < butene, until a maximum value is reached of about 95%. The reactivity towards hydrogen was found to be propene < butene < ethene.