We have found that the composition of Fe-Ni catalysts can have a profound effect on the activity for the decomposition of CO/H-2 mixtures at 600 degrees C. As the fraction of nickel in the bimetallic is increased above 70% there is a significant decline in the percentage conversion of CO. The amount of solid carbon deposited on a given bimetallic catalyst was shown to increase as the hydrogen content in the reactant mixture was raised to between 20 to 30% and then exhibited a steady decline with further addition of hydrogen. It was also evident that the structure and crystalline perfection of the carbon filaments that are produced during the reaction were very sensitive to the ratio of the two components in the catalyst. Temperature programmed oxidation and TEM studies of carbon filaments grown from various Fe-Ni bimetallic particles demonstrates that as the nickel content of the catalyst is increased there is a concomitant decrease in the crystalline perfection of the deposited carbon structures. These features are rationalized according to the notion that the addition of nickel to iron results in a reconstruction of the particle surfaces that leads to a diminution in the likelihood of iron atoms being nearest neighbors and the generation of atomic arrangements at carbon precipitating faces that do not favor the formation of graphite platelets in the deposited filamentous structures. A reversible deactivation process was shown to exist for all these Fe-Ni catalysts. At temperatures of about 725 degrees C, the growth of carbon filaments suddenly ceased; however, restoration of catalytic activity could be readily achieved by lowering of the temperature to a previously active regime. Possible reasons for this unusual behavior are presented.