The effect of adding tin to a cobalt catalyst has been investigated using the decomposition of ethylene at 600 degrees C to form solid carbon, methane, and ethane as a probe reaction. These experiments have been carried out in a simple continuous flow system, where it was possible to monitor not only the gaseous product distribution but also the growth of filamentous carbon as a function of time. Particular attention was given to the modifications in the structural characteristics of the carbon filaments that accompanied the change in catalyst composition. These studies were performed using a combination of temperature-programmed oxidation, BET surface area measurements, and examination at high resolution in the transmission electron microscope. It was found that the introduction of as little as 0.5 wt % of tin into cobalt produced a dramatic increase in the catalytic activity of the bimetallic toward the formation of filamentous carbon. It is suggested that tin is responsible for inducing electronic perturbations in the surface of cobalt, and this phenomenon is reflected in a major change in the dissociative chemisorption behavior of ethylene on the mixed metal catalyst. Increasing the concentration level of tin did not lead to any further enhancement in the amount of carbon generated from the reaction; however, this procedure resulted in a significant increase in the rate of catalyst deactivation. It was also found that in the presence of tin the filamentous carbon structures acquired a high degree of crystalline order.