The kinetics of the reactions of selected secondary and tertiary peroxy radicals (RO2) with CH3C(O)O2 have been investigated. Values of (1.0¿0.3)¿10-11, (1.1¿0.3)¿10-11, (1.0¿0.5)¿10-11 and (1.1¿0.3)¿10-11 (units of cm3 molecule-1 s-1, statistical errors 2&sgr;) have been obtained at 298 K for the rate constants of the reactions of CH3C(O)O2 radicals with c-C6H11O2, sec-C10H21O2, sec-C12H25O2 and t-C4H9O2 radicals, respectively. A systematic propagation of error analysis has yielded overall (statistical+systematical) uncertainty factors of 1.6, 1.8, 1.9, and 1.5, respectively, for the above rate constant values. The present results, combined with the results previously reported for primary peroxy radicals, show that all cross reactions of CH3C(O)O2 are fast with rate constants of around 1.0¿10-11 cm3 molecule-1 s-1, independent of the RO2 radical structure and of its self-reaction rate constant. This suggests that all acylperoxy radicals present the same high reactivity as CH3C(O)O2, and hence it is proposed to assign the above rate constant value to all cross reactions of acylperoxy radicals. These new rate constant values were implemented in the Regional Atmospheric Chemistry Mechanism <Stockwell et al., 1997> to estimate the importance of the cross reactions in the chemistry of acylperoxy radicals in the troposphere. In the case of a moderately polluted troposphere, under low NOx and high volatile organic compounds (VOC) concentrations, the cross reactions of acylperoxy radicals with organic peroxy radicals account for more than 20% of the acylperoxy loss reactions, and peroxyacyl nitrates concentrations decrease by more than 4%, compared with the previous estimates of Kirchner and Stockwell <1996>. ¿ 1998 American Geophysical Union