Cavity ringdown spectra of the (A) over tilde (2)A'-(X) over tilde (2)A " electronic transition in the IR are reported for the methyl and ethyl peroxy radicals. Analysis of partially resolved rotational structure for the origin band of the transition provides information about both the (A) over tilde and (X) over tilde states of CH3O2.. An estimate for the absorption cross section is determined from the CRDS absorption and the rate of radical-radical recombination.