The rate coefficients for the gas-phase reactions of allyl-, tert-butyl-, cyclopentyl-, and 2-pentylperoxy radicals with NO have been measured at 297 +/- 2 K in a flow tube reactor using chemical ionization mass spectrometric (CIMS) detection of the peroxy radical. The hydrocarbon radicals were produced through the dissociation of the parent alkyl iodide in a low-power radio frequency (rf) discharge. The unimolecular decomposition of the c-pentyl radicals in the rf discharge yielded allyl radicals, The peroxy radicals were generated by reacting the hydrocarbon radicals with O-2. The rate coefficients were found to be, in units of 10(-12) cm(3) molecule(-1) s(-1), 10.5 +/- 1.8, 7.9 +/- 1.3, 10.9 +/- 1.9, and 8.0 +/- 1.4 for the reactions of NO with CH2=CHCH2O2, t-C4H9O2, C-C5H9O2, and 2-C5H11O2 radicals, respectively. The results of this study together with our previous results for nonsubstituted C-1-C-3 alkyl peroxy radicals suggest no significant trend in the rate coefficients with size and branching of the radicals, This is in contradiction to some previous studies, which found that the rate coefficients decrease with increasing radical size and complexity. Some implications of this finding for atmospheric chemistry are briefly discussed.