The (A) over tilde - (X) over tilde electronic absorption spectrum of allyl peroxy radical has been recorded at room temperature by cavity ringdown spectroscopy. Photolysis of an allyl bromide/O-2 mixture at 193 nm or 248 nm yields allyl peroxy and HO2 radicals, with 248 nm photolysis more strongly favoring allyl peroxy formation. Utilizing electronic structure calculations ( including computed OOCC torsional potentials), the experimental spectrum is assigned in terms of the three conformers predicted to have the greatest intensity. This work represents an extension of recent progress in measuring (A) over tilde - (X) over tilde absorption spectra of saturated organic peroxy radicals to unsaturated species. (C) 2010 Elsevier B. V. All rights reserved.