A scanning tunneling microscope that operates inside an atmospheric pressure chemical reactor cell has been used to study the structure of a Pt(111) model catalyst surface during the thermal decomposition of propylene (coking). Carbonaceous clusters were produced by partial dehydrogenation and polymerization of the hydrocarbon, Flat (1 and 2 Angstrom high) clusters formed with lateral dimensions of several tens of angstroms, The clusters were found to be mobile when the degree of dehydrogenation was low but became immobile with more extensive dehydrogenation, The amount of dehydrogenation and the structure of the hydrocarbon cluster depended upon annealing temperature (in the range from 300 to 800 K) and gas environment (vacuum, CO, H-2/propylene).