Abstract Constable used the first title for a review in 1957. But he did not regard the influence of kinetics. The ignorance of kinetics is still a problem of immediate interest. Constable introduced the concept of “active site distribution model” to explain the variation of the “experimental” activation energy caused by thermal deactivation. He used true kinetic terms for the interpretation although he observed apparent kinetic terms. This inconsistency can be resolved with an “apparent” particle size effect, due to the variation of the number of sites.