We study the propagation pattern of a diffracting optical beam in a moving nonlinearly absorbing and chemically active gas mixture. The case of absorption of light energy by a reaction product is analyzed. It is demonstrated that the dependence of concentration of the reaction product on the input beam intensity is bistable in character, as in stationary medium. This property of interaction manifests itself through the formation of a high-absorption region. The absorption of light energy by the reaction product intensifies the effect of thermal defocusing with increasing shift of the beam center counter to the direction of motion of the medium. The intensity distribution over the beam cross section, which is initially Gaussian, may be transformed into the distribution with two maxima upon exiting the medium.