Thermal conductivity of superconducting YBa 2Cu 3O 7−δ has been measured in air and oxygen atmospheres in the temperature range 315–1150 K using an axial heat flow comparative apparatis. Measurements were also carried out on tetragonal YBa 2Cu 3O 7−δ in argon atmosphere. The thermal conductivity of the superconducting sample showed a plateau up to ∼ 650 K in air and up to ∼ 740 K in oxygen atmosphere; thereafter the conductivity increased linearly with temperature with a change of slope at ∼ 925 K in the former case and at ∼ 975 K in the latter. No plateau has been observed for the tetragonal sample measured in argon. Different possible modes of heat conduction in oxides are reviewed and the observed behaviour is found to be similar to that in amorphous and disordered systems.