A great number of inorganic models for catalase or peroxidase activity have been developed by KRAUSE et al.1)-5). Of one of these6), consisting of basic Magnesiumcarbonate (3 MgCO3 · Mg(OH)2 · 3 H2O) at the surface of which Co++ ions are adsorbed, the kinetics of catalase action were studied by him. In a reaction mixture, containing 10 mg carrier and 1 mg cobaltous ions beside 150 ml 0.3 % H2O2, at 37 °C, the hydrogenperoxide disappears according to a first order reaction. We have found that this catalase model shows a peroxidase activity as well. For the peroxidase reaction, mesidine was chosen as the H-donor6). 0.010 g basic magnesiumcarbonate is brought together during 15 min with 1 ml CoCl2 solution (1 mg Co++/ml). After this, 9 ml veronal-acetate-HCl buffer (veronal, 0.064 M; acetate, 0.064M; HCl, 0.0362M), 2 ml mesidinehydrochloride solution (0.1 M) and 20 ml H2O2 solution (0.0808 M) are added. The pH of this reaction mixture is 7.6. At definite intervals, 2 ml samples are withdrawn and added to 3 ml of a saturated solution of TiOSO4 in 2N H2SO4. The anil, resulting from the mesidine by peroxidase activity of the model system, is extracted immediately with 5 ml benzene and determined spectrophotometrically in this phase (λmax = 495 mμ; ε495 mμ = 1.185 lit Mole⁻¹ · cm⁻¹). We apply the formation of the yellowish peroxo-disulphato-titanium (IV)-ion by the reaction of titaniumsulphate with hydrogenperoxide as to determine simultaneously the H2O2 concentration in the reaction mixture. The reaction is seen to start with a short induction period, after which the anil is formed with a constant rate (zero order reaction) (Fig. 1). Further on the reaction velocity decreases to zero. The anil concentration present in the reaction mixture becomes constant as there is no longer any H2O2 available. This maximal amount of anil indicates a use of 1-4% of the total amount of H2O2 by peroxidase action. The catalase activity thus seems to proceed undiminished and contributes for the consumption of the other part of the H2O2. Due to the use of buffer, however, the reaction kinetics of the catalase activity are different from those stated by KRAUSE 0,06' and PLURA2); the decomposition of H2O2 shows an induction period, a period of zero order reaction and ends as a first order reaction (Fig. 2). When we start the model-catalase reaction with a small concentration of H2O2 (0.00373 M) the induction period is followed immediately by a first order reaction. The reaction mechanism of the catalase action, proposed by KRAUSE 2), is based on the N-type semiconductivity properties of the model system. This accounts for the formation of radicals (e.g. OH·, HO2·, H·) on its surface, which are not consumed in a further reaction with H2O2: OH· + H2O2 → HO2· + H2O; HO2· + H2O2 → OH· + H2O + O2. Our experiments are performed in buffer. If we suppose that buffer components are adsorbed on the carrier and diminish its effective surface, then we can suggest that the radical formation occurs much slower than it is the case in the absence of buffer (induction period). By this reduced surface also, the reaction rate at high H2O2 concentrations is limited to a constant level (zero order reaction). The reaction mechanism of the model-peroxidase-activity is not yet explained, but in view of the possibilities of the system the anil very likely arises from a reaction sequence involving free radicals.