The catalytic properties of a series of unsupported copper-iron powders of varying composition were investigated by using a probe reaction of ethylene and hydrogen at temperatures over the range 450 to 800 degrees C. Analysis of both the gas-phase products and the solid carbon, mainly in the form of filamentous structures, produced during the reaction provided an in situ method for monitoring the changes in the nature of the bimetallic surfaces under reaction conditions. Although these metals exhibit only limited miscibility in each other the reaction between ethylene and hydrogen over the bimetallic powders resulted in higher carbon yields and altered selectivities to gas-phase products compared to that observed with the single components under the same conditions. The addition of only 2 wt% copper to iron caused a 20-fold increase in the amount of carbon filaments formed compared to that produced on pure iron at 600 degrees C. This behavior cannot be rationalized simply by an increase in metal dispersion; instead, one may speculate that the presence of a small amount of copper is capable of inducing electronic perturbations in the iron atoms, which results in a modification in the chemisorption characteristics of ethylene on such a surface. The presence of hydrogen in the gaseous reactant was found to facilitate the interaction between the olefin and the bimetallic surface, resulting in increased yields of both solid carbon and ethane. It is suggested that during the interaction of hydrocarbons with metals the particles adopt a morphology in which there are at least two distinct faces; a set which are only capable of adsorbing and decomposing hydrocarbons, and others that will precipitate dissolved carbon in the form of graphite platelets. Under such circumstances there are always a set of faces that are completely free of deposited carbon and available for continued catalytic action.