The energy transfer between matter and radiation at very high temperatures is investigated. A previously presented formalism is applied to a mixture of electrons, positrons, and photons interacting through Compton scattering and pair creation and annihilation. The relaxation time toward thermal equilibrium and the bulk viscosity of the mixture are derived, and numerical results are presented for temperatures 3×108K≤T≤1011K. For temperaturesT≳8×108K pair creation and annihilation is found to be more efficient than Compton scattering in transferring energy between matter and radiation.