We have carried out a comprehensive investigation of the decomposition of a C2H4/CO/H-2 reactant mixture over a series of Fe-Ni catalysts. Detailed analysis of both the gaseous products and the amount of solid carbon deposited have revealed that co-adsorption of the two carbon-containing gases produces major modifications in the behavior of the bimetallic surfaces. It was evident that the addition of CO to a C2H4/H-2 feed resulted in a substantial increase in the decomposition of the olefin over all the bimetallic powders with this effect being most pronounced on the iron-rich systems. The major product from this series of Fe-Ni catalysts was found to be solid carbon in the form of various filamentous structures. The possibility that adsorption of CO was responsible for inducing perturbations in the electronic properties of the bimetallic surfaces is considered to be a contributory factor to the change in the catalytic action. A reversible deactivation/reactivation phenomena was shown to exist for iron-rich bimetallic catalysts that was observed when either CO or C2H4 nas removed and subsequently re-introduced into the reactant feed. It was interesting to find that under the same conditions this behavior did not prevail with nickel-rich bimetallic catalysts, which merely exhibited the traditional irreversible deactivation. (C) 2000 Academic Press.