Experiments on the activity of artificial salt mixtures showed that the presence of ferric as well as ferrous salts greatly influences the catalase action in mineral water in acidic side The decomposition is conditioned by the individual or simultaneous actions of FeII and FeIII Cu, Mo, W and pentacyanoammine-ferroate ions activate iron salt. Such anious as I', Br', F, ' and PO1''' check or reduce catalase activity in iron salt, while other elements have no influence upon its activity. This activating action is considered to be cocatalastic and inactivating action to be anticatalatic, because these activating or inactivating elements decompose itself not the hydrogen peroxide in acidic side. Particularly pentacyanoammine ferroate indicates a strong cocatalatic action. This leads us to expect that the aging action of hot springs will contribute to the decompositiou of unstable iron complex salts possessing such cocatalatic action by light or heat. Moreover we can indicate that artificial solutions have the same or stronger catalatic action as natural mineral waters.