A time-dependent uptake of acetone into sulfuric-acid solutions was observed by means of a wetted-wall flow reactor coupled to a photoionization mass spectrometer. The uptake was found to be reversible, and no reactive loss of acetone was observed. Henry's law constants of acetone into sulfuric-acid were obtained from the time-dependent uptake behavior observed for 50--79 wt% of H2SO4 solution at temperatures between 230--270 K. It was found that the solubility of acetone was mainly governed by its protonation in the acid. The observed Henry's law constant would unlikely be high enough to remove acetone efficiently from the gas phase in the upper troposphere; in addition, the reaction probability due to a sulfate-mediated condensation was found to be too low. ¿ 2000 American Geophysical Union