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We have used variable temperature, ultrahigh vacuum scanning tunneling microscopy (STM), in both static and time-dependent experiments, to study the chemistry of the ethylene/Pt(111) system. Images of ethylene which exhibit long-range order have been obtained at a sample temperature of 160 K. The conversion of ethylene to ethylidyne has been observed directly in STM images. This conversion reaction is observed to occur in a ‘‘patchy’’ manner across the surface at saturation coverage. As the reaction proceeds, well-defined islands of unreacted ethylene continue to be clearly observed. Further dehydrogenation of the ethylidyne formed from ethylene leads to carbon containing particles dispersed randomly across the sample. After annealing the ethylidyne covered sample to 500 K, the surface is uniformly covered with carbon containing particles which exhibit a bimodal distribution of heights (one and two atomic layers) consisting of an average of ten and twenty carbon atoms, respectively. Further annealing to 700 K results in the formation of larger particles which appear to be a single atomic layer high and 10–15 Å in diameter in the STM images. These particles contain an average of thirty-four carbon atoms. Pt steps do not show any preferential reactivity for these reactions. Annealing the carbon particle covered surface to higher temperatures results in the formation of monolayer thick graphite islands, which eventually accumulate at the Pt steps.