Structures and energetics of complexes of HO2 with glyoxal, methylglyoxal, methylvinyl ketone, and methacrolein have been calculated using density functional theory. The hydroperoxy radical was found to form strong hydrogen bonds with glyoxal, methylglyoxal, methylvinyl ketone, acrolein, and methacrolein. Binding energies D-o for these complexes were as high as 10.9 kcal mol(-1) for HO2-methylvinyl ketone. Vibrational frequencies are presented for monomers and complexes. Equilibrium constants for the formation of some of these complexes are also calculated. As a result, it is predicted that the strongest of these complexes, the hydroperoxy radical-methylvinyl ketone complex, should exist in the atmosphere under certain conditions.