The catalytic decomposition of ethylene over iron-nickel to produce methane, ethane, and solid carbon has been shown to be extremely sensitive to the composition of the bimetallic particles and the temperature at which the reaction is performed. The addition of an extra 5% nickel to an Fe-Ni (3 : 7) catalyst was found to result in a dramatic increase in the conversion of ethylene from 13.0 to 52.0% and a corresponding increase in the amount of solid carbon, identified as filamentous in nature, from 4.6 to 38.5%. It was also established that the graphitic content of the deposit exhibited a significant increase between the compositions Fe-Ni (3:7)and Fe-Ni (25 :75). An improvement in the degree of crystalline perfection of the carbon filaments generated from the whole composition range of bimetallics was evident as the hydrogen content of the reactant mixture was raised. While increasing the temperature from 600 to 725 degrees C enhanced the decomposition of ethylene over all the Fe-Ni powders, this effect was most striking over those bimetallic particles containing 70% or less of nickel. This behavior is to be contrasted with that found in an analogous study that demonstrated that the reverse dependence held when the same set of bimetallics was reacted in CO. The phenomenon appears to be related to preferential segregation of nickel to the gas/particle interface at higher temperatures; a condition that tends to favor the decomposition of C2H4, but has the opposite effect on the decomposition of CO.