Probe molecules designed to generate methylene (CH2) were added to rhodium-, ruthenium-, and cobalt-catalysed Fischer-Tropsch reactions to investigate the mechanism of hydrocarbon chain growth during CO hydrogenation. Statistical incorporation of (CH2)-C-13 derived from (CH3NO2)-C-13 or (CH2N2)-C-13 occurred during the hydrogenation of(12)CO over Co/SiO2 catalysts (1 atm, 523 K) to give the isotopically mixed alkenes,C-13(x) (Cn-xH2n)-C-12 and the alkanes derived from them. These results show that there is complete scrambling of C-12 and C-13 labels over cobalt; the levels of (CH2)-C-13 incorporation from the probe are consistent with a process which involves the participation of methylene groups in the chain propagation step. The data are also consistent with the alkenyl cycle proposed earlier for the Fischer-Tropsch reaction. By contrast, the same probe experiments over Rh/SiO2 or Rh/Ce/SiO2 catalysts gave the isotopically distinct (CnH2n)-C-12 (from (CO)-C-12 hydrogenation) and (CnH2n)-C-13 (from oligomerisation of the (CH2)-C-13 from the (CH3NO2)-C-13 or (CH2N2)-C-13 probes), and there was very little of the isotopically mixed alkenes, C-13(x) (Cn-xH2n)-C-12 and the alkanes derived from them. Similar reactions over Ru/SiO2 exhibited behaviour intermediate between cobalt and rhodium. Significant amounts of substituted amines and nitrile compounds are additionally formed when nitromethane is used as a probe;C-13 incorporation into the nitrogenous products was observed when (CH3NO2)-C-13 was used as probe. The relative ability of nitromethane to produce N-containing compounds decreases in the order rhodium > ruthenium > cobalt. There was little C-13 incorporation into the oxygenates (methanol, ethanol, and acetaldehyde) when (CH2N2)-C-13 or (CH3NO2)-C-13 was used as a probe.