The kinetics of The reactions of selected peroxy radicals (RO(2)) with CH3O2 and with C2H5O2 have been investigated using two techniques : excimer-laser photolysis and conventional flash photolysis, both coupled with UV absorption spectrometry. Radicals were generated either by photolysis of molecular chlorine in the presence of suitable hydrocarbons or by photolysis of the appropriate alkyl chloride. All such cross-reaction kinetics were investigated al 760 Torr total pressure and room temperature except for the reaction of the allylperoxy radical with CH3O2, for which the rate constant was determined between 291 and 423 K, resulting in the following rate expression : k(15) = (2.8 +/- 0.7) x 10(-13) exp[(515 +/- 75)/T] cm(3) molecule(-1) s(-1). Values of (2.0 +/- 0.5) x 10(-13), (1.5 +/- 0.5) x 10(-12), (9.0 +/- 0.15) x 10(-14), <2.0 x 10(-12), (2.5 +/- 0.5) x 10(-12), and (8.2 +/- 0.6) x 10(-12) (units of cm(3) molecule(-1) s(-1)) have been obtained for the reactions of CH3O2 radicals with C2H5O2, neo-C5H11O2, c-C6H11O2, C6H5CH2O2, CH2ClO2, and CH3C(O)O-2, respectively, and (1.0 +/- 0.3) x 10(-12), (5.6 +/- 0.8) x 10(-13), (4.0 +/- 0.2) x 10(-14), and (1.0 +/- 0.3) x 10(-11) (units of cm(3) molecule(-1) s(-1)) for the reactions of C2H5O2 with CH2=CHCH2O2, neo-C5H11O2, c-C6H11O2, and CH3C(O)O-2 radicals, respectively. These rate constants were obtained by numerical simulations of the complete reaction mechanisms, which were deduced from the known mechanisms of the corresponding peroxy radical self-reactions. A systematic analysis of propagation of errors was carried out for each reaction to quantify the sensitivity of the cross-reaction rate constant to the parameters used in kinetic simulations. The rate constant for a given cross reaction is generally found to be between the rate constants for the self-reactions of RO(2) and CH3O2 for C2H5O2). However, when the RO(2) self-reaction is fast, the cross reaction with CH3O2, (or C2H5O2) is also fast, with similar rate constants for both reactions, suggesting that these particular peroxy radical cross reactions can play a significant role in the chemistry of hydrocarbon oxidation processes in the troposphere and in low-temperature combustion. Relationships between cross-reaction and self-reaction rate constants are suggested.