General equations are derived whose solution gives the dependence of species concentrations of a closed gaseous system on time for a given dependence of temperature on time, on the assumption that either the dependence of pressure in on time or the dependence of volume of system on time is given. Some results from solving a previously proposed model are given for the case when is created by the products of dissociation and ionization and the temperature decreases exponentially from 10 000 K to 2500 K. Special attention is devoted to the relation between time-dependent composition and equilibrium composition.