Key substances are such that the concentrations of all other substances involved in the reaction are a lin� ear function of the set of these substances. A basic property of the set of differential equations of chemi� cal kinetics is that the number of existing independent linear laws of concentration conservation is no less than the number of chemical elements constituting the substances involved in the reaction [2]. Information on key substances is of crucial impor� tance in solving the inverse problems of identification of mechanisms of complex chemical reactions. A major challenge in solving such problems is insuffi� cient information provided by measurements [3, 4]. Hypothetical schemes of reaction mechanisms involve a large number of substances and reactions between them. Mathematical descriptions of these mecha� nisms are sets of differential equations in which the number of unknowns is equal to the number of sub� stances involved in reactions. At the same time, only some of these substances can be directly measured. There arises an inverse problem of determining the parameters of the set of differential equations (rate constants of chemical reactions) reproducing part of its solutions. A consequence of insufficient informa�