Thermogravimetric analysis of yttrium-barium cobaltite phase, recently reported as YBaCo4O7, showed a higher oxygen content at temperatures below 1000 K. This phase, equilibrated with atmospheric oxygen at room temperature, contains approximately 8.5 oxygen atoms per formula unit, suggesting that the dominant oxidation state of cobalt is +3. The high effective paramagnetic moment, mu(eff) = 5.55 mu(B) per Co ion, supports such conclusion and indicates the high-spin electronic configuration. The total conductivity of dense YBaCo4O7+delta ceramics is predominantly p-type electronic. The oxygen ion transference numbers, estimated from the data on steady-state oxygen permeation, vary from 3 x 10(-5) to 2 x 10(-4) at 1073-1223 K, increasing with temperature. The results of thermal analysis and coulometric titration, and also the P(O-2)-dependencies of total conductivity and Seebeck coefficient show that heating above 1070-1110 K in air and/or decreasing oxygen partial pressure result in a phase transition accompanied with substantial oxygen losses from the lattice, forming essentially stoichiometric YBaCo4O7. Except for these changes, the transport properties and oxygen content are almost P(O-2)-independent down to the oxygen pressures of 10(-5) atm. At temperatures below 40 K, YBaCo4O7+delta exhibits in a spin-glass like magnetic state with a weak ferromagnetic component. (c) 2005 Elsevier B.V. All rights reserved.