The reaction between the hydroperoxy radical, HO2, and acetone may play an important role in acetone removal and the budget of HOx radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215-272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO2. The HO2 concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO2] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO2] followed by subsequent decay. These curves are interpreted as a rapid (<100 mu s) equilibrium reaction between acetone and the HO2 radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 x 10(-16) to 7.7 x 10(-48) cm(3) molecule(-1) for T = 215-272 K Thermodynamic parameters for the reaction determined from a second-law fit of our van't Hoff plot were Delta H-r degrees(245) = -35.4 +/- 2.0 kJ mol(-1) and Delta S-r degrees(245) = -88.2 +/- 8.5 J mol(-1) K-1. Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO2-acetone) with subsequent isomerization to a hydroxyperoxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 mu s). Formation of the hydrogen-bonded complex will have a negligible impact on the atmosphere. However, the complex could subsequently isomerize to form the 2-HIPP radical on longer time scales. Further experimental studies are needed to assess the ultimate impact of the reaction of HO2 and acetone on the atmosphere.