We present the full solution of the master equation for the system with reversible isomerization and decomposition channels at low pressures. As an example of such a system we consider the cis-trans isomerization of 2-butene. At high temperatures cis-2-butene decomposes into butadiene and hydrogen. The effect of isomerization on the decomposition rate coefficient was studied and indicated multiple steady-state behavior. At 1200 K, for example, a true steady state is achieved only after 75% of the product has been formed. This behavior is explained in terms of relaxation to the equilibrium distribution between cis and trans isomers. The first plateau in the rate coefficient corresponds to the irreversible regime of isomerization when two isomers are far from equilibrium, while the second plateau or true steady state is established after equilibrium between isomers has been reached. The effect is not observed at either low temperatures or high pressures.