Let us consider an heterogeneous reaction in which a gas G is present with one or more solids. The mechanism of such a reaction is generally very complex, but substantial progress has been realized in this domain by studying the gas pressure dependence of the reaction rate. In the same way and as a concrete example, we have studied the pressure dependence of a gas G' different from the gas G and called the "foreign" gas of the reaction. This "foreign" gas changes the kinetics of the reaction, and the rates of the process can be increased or decreased. Such changes may be induced by different phenomena...