C-13 NMR spectroscopy shows that the n-alkene and n-alkane products from the catalytic hydrogenation of CO in the presence of (C2H4)-C-13 probes over Ru/150 degreesC, Co/180 degreesC, Fe/220 degreesC, or Rh/190 degreesC (1 atm, CO:H-2 1:1, "mild conditions") contain terminal (CH3CH2)-C-13-C-13- units. This is consistent with their formation by a regiospecific polymerization of C, species derived from CO and initiated by (C2H4)-C-13. Although the activities toward individual products differed somewhat, similar distributions and similar product labeling patterns were obtained over all the four catalysts. 1-Butene and the higher 1-n-alkenes from all the catalysts were largely (CH3CH2)-C-13-C-13(CH2)(n)CH=CH2 (n = 0-3), propene formed over Ru or Co was (CH3CH)-C-13-C-13=CH2, while both (CH3CH)-C-13-C-13=CH2 and (CH2)-C-13=(CHCH3)-C-13 were formed over Fe or Rh. Comparison of the conclusions from these probe experiments with those from isotope transient experiments by other workers indicates that the ethene initiator does not significantly modify the course of the CO hydrogenation. The reaction products are largely kinetically determined, and the primary products are mainly linear 1-n-alkenes, while the n-alkanes and 2-n-alkenes largely arise via secondary processes. Since the distribution of products and the labeling in them is so similar, it is concluded that one basic primary mechanism applies overall the four metals. Several different reaction paths involving a polymerization of surface methylene, {CH2(ad)}, have been proposed. Although the predictions based on several of these mechanisms agree with many of the results, the alkenyl + {CH2(ad)} mechanism, initiated by a surface vinyl {CH2=CH(ad)}, most easily accommodates the experimental evidence. An alternative path involving sequential addition of surface methylidyne and hydride either to a growing alkylidene chain (alkylidene + {CH(ad) + H-(ad)}) or to an alkyl chain (alkyl + {CH(ad) + H-(ad)}) has recently been proposed by van Santen and Ciobica. The {CH2(ad)} mechanism offers an easier explanation for the formation of the various alkenes, the distribution of products, and of the initiation, while the {CH(ad) + H-(ad)} mechanism can explain any n-alkanes formed as primary products and not derived from alkenes. At higher reaction temperatures over Ru and Co, considerable C-13, incorporation (from natural abundance in the CO and from cleavage of the (C2H4)-C-13 probe) was found in all the hydrocarbons. Thus, at higher temperatures C-13(1(ad)) in addition to C-13(2(ad)) species participate in both chain growth and initiation. In summary, adsorbed CO is transformed very easily into surface Cl(ad), probably {CH2(ad)} in equilibrium with {CH(ad)+H-(ad)}, which act as the propagating species.