Vinylic probes were added to a ruthenium-catalysed Fischer-Tropsch reaction in order to study the mechanism of hydrocarbon formation from CO/H-2. C-13(2) units derived from (CH2)-C-13=(CHBr)-C-13 or (CH2)-C-13=(CH2)-C-13 are readily incorporated into the alkene and alkane products formed from (CO)-C-12 and H-2 over a Ru/SiO2 catalyst (1 atm, 180 degrees C). The levels of C-13(2) incorporation suggest that the probes are sources of C-13(2)-labelled initiating units, proposed to be surface vinyls (-CH=CH2). Further evidence for vinyl initiation is given by the increase in hydrocarbon formation rate during vinylic probe addition. This rate increase is not observed when ethyl bromide is added as a probe and shows the significance of the unsaturated initiating unit. C-13(4) incorporation in the C-4 hydrocarbon products (especially cis-2-butene) is attributed to a facile vinyl dimerisation reaction, as previously observed on rhodium catalysts and in discrete organometallic complexes. Hydrogenolysis of the C-13(2)-probes was an appreciable side reaction over Ru/SiO2 and resulted in some incorporation of C-13(1) units.