The interaction between combustion and relative atmospheric-oxygen concentration during biomass burning events is investigated by the occurrence and abundance of pyrogenic polycyclic aromatic hydrocarbons (PAH) from modern, Triassic, and Carboniferous chars. The combustion process consumes atmospheric O₂ and thermally alters organic matter to produce chars. PAH that develop through combustion, and are preserved within chars, are expected to be more abundant at times of elevated atmospheric O₂, which facilitates biomass burning efficiency. To investigate this relationship, PAH have been extracted from chars of three geologic periods - modern, Triassic, and Carboniferous - which relate to 21%, 15%, and 35% atmospheric O₂, respectively (Berner and Canfield, 1989). Surface samples of modern chars that developed in controlled burns set by National Park Service personnel in a mixed conifer-deciduous forest were collected at Zion National Park. Triassic fusain from Petrified Forest National Park and Carboniferous fusain from Joggins Fossil Cliffs were collected from floodplain-deposited mudstone and sandstone. Target PAH shown to be uniquely pyrogenic include: phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and benzo(ghi)perylene. The abundance of PAH from modern chars at Zion National Park, Triassic fusain from Petrified Forest National Park, and Carboniferous fusain from Joggins Fossil Cliffs seem to reflect differences in atmospheric oxygen levels.