A variety of experimental techniques including flow reactor studies, temperature-programmed oxidation, and transmission electron microscopy have been used to investigate the manner by which silver modifies the catalytic behavior of cobalt. For this purpose the growth of filamentous carbon along with the gas phase product distribution resulting from the decomposition of ethylene over cobalt and cobalt-silver particles at 600 degrees C has been used as a probe reaction. It was found that the incorporation of as little as 1% of silver into cobalt caused a increase of over an order of magnitude in the amount of carbon filaments produced compared to that formed on pure cobalt under the same experimental conditions. It is suggested that some of the observed enhancement in catalytic activity could be due to the existence of electronic perturbations of cobalt induced by the presence of silver that are manifested in modifications in the adsorption and subsequent decomposition characteristics of ethylene on the bimetallic surface. Analysis of the gas phase effluent reveals that ethane is the major product with minor amounts of methane. This distribution indicates that on the cobalt-silver surface there is a strong tendency for ethylene to adsorb in a direction parallel to the surface and either is hydrogenated to ethane or undergoes carbon-carbon bond scission with resulting carbon species dissolving and diffusing through the catalyst particle to form the filamentous carbon deposit. This behavior is to be contrasted with that found for the cobalt-copper/ethylene system where a significant amount of methane is produced and believed to originate from the decomposition of an ethylidyne intermediate arising from adsorption of the olefin on the bimetallic surface in an "end-on" configuration.