Atmospheric concentrations of a series of carbonyl compounds known as formaldehyde (FA), acetaldehyde (AC), acetone (AN), glycolaldehyde (GA), glyoxal (GL), methylglyoxal (MG), glyoxylic acid (GD), and pyruvic acid (PD) were measured at a rural site in Georgia in summers of 1991 and 1992. The midafternoon median concentrations, in parts per billion, determined for 1991--1992 were FA, 3.6/3.1; AC, 0.58/0.74; AN, 1.7/1.8; GA, 0.21/0.26; GL, 0.02/0.09; MG, 0.03/0.08; GD, 0.46; and PD, 0.11, the latter two for 1992 only. All of the carbonyls except AC and AN exhibited a strong diurnal dependence, with maxima in the midafternoon and minima during the night, consistent with a rapid in situ photochemical production in the daytime and a loss by dry deposition in a shallow inversion during the night. FA correlated well with O3, GA, and MG, consistent with their photochemical production near the surface at the measurement site. GL and MG showed the strongest correlation among all species, suggesting common origins as well as similar atmospheric lifetimes. The presence of GA, MG, and GL along with FA at the observed relative concentrations are consistent with laboratory developed isoprene oxidation mechanism and the expectation that isoprene represents a major reactive hydrocarbon in this rural region. At the concentrations observed, these carbonyls serve as important radical sources. The contribution of FA accounts for half of that by O3 and the higher carbonyls approximates half of that by FA. With respect to production of peroxyacetyl nitrate, isoprene contributes as much as acetaldehyde. These results lend further credence to the notion that isoprene plays a pivotal role in photochemical processes, especially in rural environments. ¿ American Geophysical Union 1995