The optical absorption of reduced and doped SrTi03 has been measured between 0.39 and 7 p,m. Transport measurements were also carried out between room temperature and 10 K for various samples with different treatment. Both optical and transport properties were found to depend greatly on the methods of preparations of the samples. We found up to five absorption peaks in the visible and infrared regions, lying at 0.43, 0.52, 0.70, 1.2, and -4, 8 p, m. In all samples, the so called free-carrier absorption tail starts falling off at about 2.5 p,m and has a maximum between 4.5 and 5 p, m. The position of this peak cannot be explained by the free-carrier absorption theory of LO-mode scattering.The new absorption mechanisms are introduced in order to explain the results in this wavelength region. Of the five absorption peaks, the first, second, and fourth peaks are due to impurity or defect levels, whereas the third and fifth peaks are interpreted as being due either to interconduction-band or vacancy-level to conduction-band transitions. The conduction and scattering mechanisms are discussed.

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