Tracer studies were performed on cobalt and iron Fischer-Tropsch catalysts using a synthesis gas containing a 20:80 mixture of (13)C18O and (CO)-C-12-O-16. The objective of the work was to investigate the antecedents of the C-O bonds in alcohols and CO2 formed during Fischer-Tropsch (FT) synthesis. It was found that chain growth proceeded by a CO insertion mechanism over both cobalt and iron catalysts. Over the cobalt catalyst, the dominant pathway for methanol synthesis involved a partial hydrogenation of CO as well as CO2 by a reaction pathway separate from the Fischer-Tropsch pathway. Over the iron catalyst, the majority of the methanol was formed by partial hydrogenation of only CO through the FT reaction pathway. Iron is active for water gas shift conversion, which produced CO2. Oxygen exchange reactions of CO2 were likely over both catalysts and complicated the interpretation of the results.