Strontium titanate single crystals have been carefully reduced at temperatures ranging from 1200 to 1400°C in oxygen partial pressures of 10 -7-10 -12 atm and quenched to room temperature where optical absorption coefficients, Hall coefficients, mass densities and lattice parameters were measured. (Optical absorption coefficients and Hall coefficients were also measured at 77 K). The predominant defects at room temperature were found to be doubly ionized oxygen vacancies and conduction band electrons. No evidence was found for singly ionized oxygen vacancies as has been suggested by some workers. Optical absorption in the visible and ultraviolet frequency region was found to be dependent upon the concentration of free carriers. For photon energies of 0.5 eV to about 1 eV, a free carrier absorption effect occurs which may be due to intraband transitions. Three peaks occur at photon energies of 1.66, 2.44, and 2.95 eV and may be due to phonon assisted interband transitions among the titanium d-like conduction bands.