Expressing the amount of a catalyst is often most conveniently done by assessing its catalytic effect. Under zero-order kinetics the catalyzed rate of conversion for a specified reaction is constant and proportionate to the amount-of-substance concentration of catalyst present. Defining the kind-of-quantity "catalytic activity" as a property of the catalyst measured by the catalyzed rate of conversion, the coherent SI unit is mole per second. Following thorough discussions in the IUPAC, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), the International Union of Biochemistry and Molecular Biology (IUBMB), and the World Health Organization (WHO), a petition was presented by the IFCC to the International Committee for Weights and Measures (CIPM) on a special name "katal", symbol kat, for "mole per second" when used in measuring catalytic activity. This request was granted by the General Conference for Weights and Measures (CGPM) by Resolution 12 (1999). Hereafter, the coherent SI unit katal = 1 mol s1 should replace the off-system IUB unit "(enzyme) unit" = 1 mmol min1 16.67 nkat. Thus, by suitable definition of reaction conditions, results for catalytic activity and derivative kinds-of-quantity are metrologically traceable to the SI.