The demand for better understanding of the mechanism of soot formation is driven by the negative environmental and health impact brought about by the burning of fossil fuels. While soot particles accumulate most of their mass from surface reactions, the mechanism for surface growth has so far been characterized primarily by measurements of the kinetics. Here we provide atomic-scale scanning tunneling microscope images of carbon growth by chemistry similar to that of importance in soot formation. At a temperature of 625 K, exposure of the surface of highly ordered pyrolytic graphite to 1 Langmuir of acetylene leads to the formation of both graphitic and amorphous carbonaceous material at the edges of nanoscale pits. Given the similarity of the electronic structure at these graphite defect sites to that of soot material growing in flames at higher temperatures, the present studies shed light on the mechanism for soot growth. These experiments also suggest that healing of defect sites in graphene nanostructures, which are of considerable interest as novel electronic devices, should be possible at modest surface temperatures by exposure of defected graphene to unsaturated hydrocarbons.