Both stable conformers, trans (T) and gauche (G), of the ethyl peroxy radical and its perdeutero analogue have been observed via cavity ringdown spectroscopy (CRDS) of the (2)A'-(2)A' ' electronic transition in the near-IR. Assignments of specific spectral lines to the electronic transition origin (T-00), to observed vibrational hot bands, and to the COO bend and the O-O stretch vibrations are given with the help of equation of motion (EOMIP) quantum chemical calculations. In particular, spectral information for the previously unknown/unassigned T conformer of ethyl peroxy is given in this study for the first time and compared to the data for the previously observed G conformer. The conformer assignment is confirmed by an analysis of the partially resolved rotational structures. The electronic origins for the T and G conformers of C2H5O2 are located at 7362(1) and 7592(1) cm(-1), respectively.