We calculate exactly the equilibrium properties of a large system consisting of two kinds of Bose particles,aandb, that can be transformed into each other by an external field. The enhancement in the specific heat around the threshold (internal excitation) energy of thebparticles is studied as a function of the external field strength, and the mass–action law is derived. Bose–Einstein condensation can occur in our mixture of reacting gases at a transition temperature lower than that of a one‐component ideal gas. Also, the condensate occupancy increases faster thanT3/2as the temperature is lowered.