The reaction probability of ozone on the surface of several different organic liquids is measured. Ozone reacts efficiently (&ggr;≈10-2--10-3) with organic liquids containing double bonds (1-tridecene, &agr;-pinene, toluene, methacrolein and canola oil), and much less efficiently with an aldehyde, a ketone (tridecanal, 2-tridecanone; &ggr;≈10-4), an alkane, an alcohol and an acid (tridecane, 1-tridecanol, tridecanoic acid; &ggr;≤10-5). The results suggest that heterogeneous reactions with organic aerosol can be an important loss process for tropospheric ozone, if the aerosol contains unsaturated organic material. ¿ 1998 American Geophysical Union