A combination of flow reactor studies and electron microscopy techniques have been used to investigate the manner by which the composition of iron-nickel alloy particles influence the growth characteristics of carbon deposits formed during the decomposition of ethane at temperatures over the range 815 to 865 degrees C. Major differences in the selectivity patterns of alloys were evident with the amount of catalytically produced solid carbon being significantly higher on a Fe-Ni (5:5) powder than on a Fe-Ni (8:2) sample. Examination of the deposit revealed the existence of two types of structures, carbon nanofibers and a graphite shell-like material, both of which contained associated metal particles. The latter structures appeared to predominate at the higher temperature and were most abundant on the Fe-Ni (5:5) particles. A dramatic change in catalyst activity and selectivity was found when 50 ppm H2S was added to the ethane feed. Analysis of the gas phase product distribution showed that the behavior of the two alloy powders was almost identical. On the other hand, the yields of solid carbon were generally higher on the iron-rich sample and tended to consist of the shell-like form on both alloys when sulfur species were present in the reactant.