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The activity of a catalyst increases as the degree of dispersion of the active component increases. In the case of supported catalysts it is possible to achieve dispersion right down to single active atoms. On the other hand, the smallest individual units of pure substances which can be obtained are generally 1 — 10 nm crystals. These finely dispersed systems can be stabilized by admixture with just small quantities of foreign substances. At temperatures above 600°C certain metal catalysts are capable of undergoing spontaneous dispersion under reaction conditions. — The degree of dispersion and the specificity of a catalyst are interrelated in a complex manner. Certain so-called demanding reactions proceed most readily on finely dispersed systems (with low-indexed crystallographic faces), others on coarsely dispersed active components (with predominantly highly-indexed faces). — Although the selectivity of reactions taking place on model catalysts frequently correlates with the degree of dispersion, such a relationship has only seldom been observed in the case of technical catalysts.