It has been shown by Dash that an optical absorption band at 2.0 ev occurs in BaO crystals when these are treated in barium or other metal vapors, giving the crystals a blue color. In order to determine the way the stoichiometric excess of metal is present in the lattice, the diffusion of the blue coloration into BaO has been measured over the range 800\ifmmode^\circ\else\textdegree\fi{}C to 1300\ifmmode^\circ\else\textdegree\fi{}C. The diffusion constant has been found to obey the empirical relation, $D=2500\mathrm{exp}(\ensuremath{-}\frac{32500}{T})$. Comparison of this with Redington's results for the diffusion of ${\mathrm{Ba}}^{140}$ into BaO indicates that the diffusion process giving the coloration does not transport barium. It is concluded that the principal lattice defects in BaO with excess metal are oxygen vacancies, and the absorption band is like the ${F}^{\ensuremath{'}}$ band in alkali halides. Chemical determinations of the barium excess are compared with optical determinations.