Various relaxation stages in high-velocity and high-temperature gases with physicochemical processes are considered on the basis of model kinetic equations. Macroscopic equations are derived in the zero approximation of the modified Chapman–Enskog method and expressions for the flow members of gas-dynamic equations in terms of intensive and extensive parameters are deduced. A formula for the velocity of sound (as the velocity of propagation of small perturbations) is derived using the parameter æ, which is not a constant under the considered conditions.