The work presents the results of electrical conductivity and thermoelectric power measurements for Bi2Sr2CaCu2O8-y performed as a function of temperature (300-1100 K) and oxygen pressure (1-10(-3) atm) under thermodynamic equilibrium conditions. An analysis of the electrical properties as a function of deviation from stoichiometry enabled the determination of the type of dominant electronic defects and transport properties in a wide temperature range. The reduction of Cu3+ to Cu2+ seems to be an effective reaction related to nonstoichiometry in oxygen sublattice. (C) 2000 Elsevier Science B.V. All rights reserved.