The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO2 (R = CH3, C2H5, and CF3), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH3O2 was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N-2 diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N-2 diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH3O2 was estimated to be < 0.1. The reaction rate constants of IO + C2H5O2 and IO + CF3O2 were determined to be (14 +/-6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N-2 diluent, respectively. The upper limit of the reaction rate constant of IO with CH3I was < 4 x 10(-14) cm(3) molecule(-1) s(-1).