The role of strongly adsorbed hydrocarbon deposits in reforming catalysis on a series of flat, stepped, and kinked platinum single-crystal surfaces at atmospheric pressures and temperatures between 300 and 700 K has been established and a model developed for the working structure and composition of the active catalyst surface. Restart reaction studies and reaction rate studies using platinum surfaces precovered with carbonaceous overlayers containing carbon-14 were used to investigate the catalytic activity and selectivity of carbon-covered platinum in hydrocarbon hydrogenation, dehydrogenation, and skeletal rearrangement. Quantitative hydrogen thermal desorption studies were carried out as a function of surface structure and reaction temperature to determine (1) the composition, and (2) the energetics for sequential dehydrogenation of carbonaceous deposits derived from a variety of adsorbed hydrocarbons including isobutane, neopentane, n-hexane, and cyclohexene. Carbon monoxide adsorption thermal desorption methods were developed to titrate uncovered platinum surface sites before and after reaction rate studies. The uncovered-site concentrations were correlated with the total surface carbon coverage as determined by Auger electron spectroscopy and the catalytic activity and selectivity of carbon-covered platinum. These experiments together with results from related structure sensitivity, thermal desorption, deuterium exchange, and radiotracer studies revealed that the primary role of the disordered carbon deposit is that of a nonselective poison which blocks platinum surface sites from incident reactant molecules. The most important chemical properties of the carbonaceous deposit are its abilities to store and exchange hydrogen with reacting surface species and to provide desorption sites for product molecules. The growth mechanism of this carbonaceous deposit is sensitive to the structure of the reacting hydrocarbon, and its morphology appears to vary continuously from two dimensional at low reaction temperatures (<550 K) to three dimensional for temperatures higher than about 600 K.