The rate constants for the gas-phase reactions of isopropyl- and tert-butylperoxy radicals with nitric oxide (NO) have been studied at 298 ± 2 K and a total pressure of 3-4 Torr (He buffer) using a laser flash photolysis technique coupled with a time-resolved negative-ionization mass spectrometry. The alkyl peroxy radicals were generated by the reaction of alkyl radicals with excess O-2, where alkyl radicals were prepared by laser photolysis of several precursor molecules. The rate constants were determined to be k(i-C3H7O2 + NO) = (8.0 ± 1.5) x 10(-12) and k(t-C4H9O2 + NO) = (8.6 ± 1.4) x 10(-12) cm(3) molecule(-1) s(-1). The results in combination with our previous studies are discussed in terms of the systematic reactivity of alkyl peroxy radicals toward NO.