In order that a reaction be thoroughly understood, it is necessary to know the concentrations of the reactants. Obtaining such knowledge is not considered a great hurdle in the investigation of homogeneous reactions. With heterogeneous reactions the question of the concentration of the reactant is not always readily answered and, as will be shown, the problem is particularly acute in heterogeneous catalysis, It is necessary to consider the catalyst itself as a reactant in at least one step of the overall reaction. Considering the catalyst a reactant is not only necessary but also fruitful when it is realized that in many cases the catalyst provides the only path whereby a reaction can occur; in such cases it is not merely a substance which increases the rate of a reaction that would occur anyway.