A new heterogeneous reaction, namely the sulfate-mediated conversion of acetone to 4-methyl-3-penten-2-one and trimethylbenzene, is reported. The reaction products were observed in their absorbed states by Fourier transform infrared spectroscopy and in the gas phase by mass spectrometry. In addition, protonated, absorbed acetone was observed under certain conditions. The reaction exhibits a strong concentration and/or temperature dependence, with non-reactive uptake as protonated acetone dominating at low temperatures and sulfate concentrations and reactive uptake occurring above 200 K and 75 wt. % H2SO4. Under conditions where reactive uptake occurs, the steady-state acetone loss probability is roughly 40%. It was not possible in this study to assess the relative effects of temperature and concentration on the reactivity of sulfate toward acetone. ¿ 1998 American Geophysical Union