The interaction between the two components of a relativistic binary gas is investigated. Starting from the relativistic Boltzmann equation, general expressions for the rate of energy exchange, the relaxation time, and the coefficient of bulk viscosity are obtained. As a particular example, the formalism is applied to the study of a gas at very high temperatures,T<∼6×109 °K, assuming that the energy exchange between matter and radiation is caused by Compton scattering; relativistic effects are fully taken into account.