The (A) over tilde-(X) over tilde electronic absorption spectrum of propargyl peroxy radical has been recorded at room temperature by cavity ring-down spectroscopy Electronic structure calculations predict two isomeric forms acctylenic and allenic with two stable conformers for each The acetylenic trans conformer with a band origin at 7631 8 +/- 0 1 cm(-1) is definitively assigned on the basis of ab initio calculations and rotational simulations and possible assignments for the acetylenic gauche and allenic trans forms are given A fourth form allenic cis is not observed Simulations based on calculated torsional potentials predict that the allenic trans form will have a long, poorly resolved progression in the OOCC torsional vibration consistent with experimental observations