A novel technique has been developed for the detection of peroxy radicals in order to study their kinetics with NO2. Peroxy radicals (RO2, where R = H, CH3, and 1,2-C4H9) were produced by laser flash photolysis and were probed by photodissociation of the RO2 and the subsequent detection of either OH or CH3O photofragments by laser-induced fluorescence. Reaction 1, CH3O2 + NO2 + M (sic) CH3O2NO2 + M (M = N-2), was studied between 25 and 400 Torr at 295 K, giving results in excellent agreement with the literature. At temperatures between 333 and 363 K, equilibration was observed and yielded Delta H-r(298)circle minus (1) = -93.5 +/- 0.3 kJ mol(-1). Reaction 2, HO2 + NO2 + M (sic) HO2NO2 + M (M = N-2), was studied at 295 K and showed kinetics in fair agreement with the literature. Equilibration at higher temperatures was obscured by an additional loss of HO2NO2 from the system. In addition, the OH quantum yield from photolysis of HO2NO2 at 248 nm was determined to be 0.15 +/- 0.03. Reaction 3, 1,2-C4H9O2 + NO2 + M (sic) 1,2-C4H9O2NO2 + M (M = He), was studied between 241 and 341 K, and at the higher temperatures equilibration was observed, which yielded Delta H-r(298)circle minus (3) = -93.5 +/- 0.6 kJ mol(-1). The low uncertainties in the enthalpies of formation for both CH3O2 and 1,2-C4H9O2 are a result of using a master equation method that allows global analysis of all the available rate data (present measurements and literature values) for forward and reverse reactions under all conditions of temperature and pressure.