Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) were used to identify the molecular species formed upon the reaction of hydrogen with surface carbon that is deposited by exposing acetylene to a Pt(111) surface held at 750 K. At this temperature, the acetylene is completely dehydrogenated and all hydrogen is desorbed from the surface. Upon subsequent hydrogen exposure at 85 K followed by sequential annealing to higher temperatures, ethylidyne (CCH3) ethynyl (CCH), and methylidyne (CH) are formed. The observation of these species indicates that carbon atoms and C-2 molecules exist as stable species on the surface over a wide range of temperatures. Through a combination of RAIRS intensities, hydrogen TPD peak areas, and Auger electron spectroscopy, quantitative estimates of the coverages of the various species were obtained. It was found that 79% of the acetylene-derived carbon was in the form of C-2 molecules, with the remainder in the form of carbon atoms. Essentially all of the acetylene-derived carbon could be hydrogenated. In contrast, 85% of an equivalent coverage of carbon deposited by ethylene exposure at 750 K was found to be inert toward hydrogenation.