Cavity ringdown spectra of the near-IR (A) over tilde (2)A' - (X) over tilde (2)A" electronic transition of the acetyl peroxy radical and its perdeutero analogue are reported. The electronic origin for the CH3C(O)O-2 is observed at 5582.5(5) cm(-1). Extensive ab initio calculations have been carried out to predict frequencies and help assign the electronic origin, observed vibrational hot bands, and several A state vibrational frequencies. An empirical absorption cross section for the origin of the (A) over tilde (2)A' - (A) over tilde (2)A" transition is estimated at sigma(CH3C(O)O2) (1+/-0.5) x 10(-1)9 cm(2).