Optical absorption studies of single crystals of BaO indicate the presence of absorption bands at 0.8 and 1.4 ev, induced by ultraviolet or x-ray irradiation at about - 160°C. These bands are enhanced about five-fold upon heating a crystal in air to about 1600°C and quenching. Crystals heated in barium vapor have a blue color and have an absorption maximum at 2.0 ev. Crystals heated in aluminum, magnesium, and calcium vapors have similar absorption curves, suggesting that either interstitial barium or oxygen vacancies are responsible for the 2.0-ev absorption. These results, taken together with the results of Redington on diffusion of radioactive barium and of Sproull, Bever, and Libowitz on the diffusion of color centers in BaO crystals, favor the latter imperfections as the cause of the coloration. Photoconduction studies, used mainly as a sensitive measurement of optical absorption, show the presence of energy levels at 2.0 and 2.6 ev in addition to those at 0.8 and 1.4 ev. Extensive x-ray irradiation greatly increases the 2.0-ev level photoconduction relative to that of the other bands.