The C-13-tracer results from the introduction of (C2H4)-C-13 into syngas prior to conversion with a rhodium catalyst have been used to support a surface vinyl mechanism for Fischer-Tropsch synthesis. The results were first interpreted by a mechanism that involved a decrease in C-13 species on the surface as the carbon number increased. This model is shown to be incorrect. Considering only the C-13-labeled products, the data are consistent with earlier tracer studies showing that the added (C2H4)-C-13 initiates chains. (C) 2000 Elsevier Science B.V. All rights reserved.