In this contribution, an optical in-situ study is reported in which the defect-induced absorption in 10 mol% Y-doped BaCeO3 was measured at various temperatures and oxygen partial pressures. Strong influences on the optical absorption were observed upon reduction. Aside from the influence of structural disorder on the band tail density of states, electronic absorption in the visible region was determined as a consequence of the hopping process of electrons with a hopping energy of 0.74 eV and a formation energy of 1.26 eV. For the first time, the electron concentrations are determined in connection with the oscillator strength of the electron hopping at various temperatures and oxygen partial pressures. The p(O2)-dependence of the electron concentration is discussed in the framework of defect chemistry. For electron holes, the results of this study strongly suggest a band conduction mechanism.