Low-level helicopter flights were used to collect samples of smoke from burning chaparral in southern California and over a boreal forest fire in norther Ontario, Canada. The smoke plume samples were analyzed for carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), methane (CH4), total nonmethane hydrocarbons (TNMHC), and nitrous oxide (N2O). Samples of gas were also analyzed from upwind of the burns to determine the ambient background levels. CO2-normalized emission ratios (ΔX/ΔCO2; where X is equal to each gas; vol/vol) were determined for each trace gas for conditions of flaming, mixed, and smoldering combustion. The emission ratios for these trace gases were found highest during smoldering combustion, generally thought to be the least efficient combustion stage. However, indications that high emission ratios for these trace gases can be produced during very vigorous flaming combustion are also indicated by our results. Whether this results from quenching processes due to rapid entrainment of air or from localized pockets of oxygen-depleted air, or both, deserves further study. Emission ratios determined for ethane (C2H6), ethylene (C2H4), propane (C3H8), and propylene (C3H6), obtained over an earlier chaparal fire at the same approximate site, are presented for the first time. While emission ratios for these hydrocarbons ranged from a factor of 15 to 100 less than their corresponding methane emission ratios, they strongly supported the production of highly reduced trace gases during vigorously flaming combustion: that is, ethylene was generated in higher proportion than ethane, and propylene in higher proportion than propane. ¿ American Geophysical Union 1989