Pyruvic acid, a product of the atmospheric oxidation of cresols and probably of isoprene, has been determined together with formic acid in atmospheric aerosols and rain as well as in the vapor phase. Both acids are present predominantly as vapor; only about 10--20% of the total atmospheric pyruvate and 1,2% of the total formate are in the particulate phase. The concentrations of pyruvic and formic acid are highly correlated, with typical formic-to-pyruvic ratios of 10--30 in the gas phase, 20--30 in rain, and 2--10 in aerosols. The gas-phase and rain ratios are comparable to those predicted to result from isoprene oxidation. Pyruvic acid levels were similar in the eastern United States (during summer) and the Amazon Basin, suggesting that natural processes, particularly the photochemical oxidation of isoprene, could account for most of the pyruvic acid present in the atmosphere. ¿American Geophysical Union 1987