The perturbation solution of the Boltzmann equation in a dilute gas shows that the rate constant of chemical reaction A + A reversible arrow B + B is diminished due to nonequilibrium effects. For the line-of-centers model, the relative decrease eta of the reaction rate can reach even nearly 45% (for a small reduced threshold energy epsilon* and a large molar fraction chi (B)). For slow reactions this method is valid and eta does not depend on chi (B). The appropriate equilibrium expressions for reaction rate (with: (a) the temperature of the system T, (b) the nonequilibrium Shizgal-Karplus temperatures T-A and T-B) are used for this analysis.