Abstract: A process and a reagent for the determination of ions in fluids, wherein the influence of these ions on the activity of an enzyme is measured. The ions for example are sodium, potassium, calcium, magnesium, manganese, lithium, lead, zinc, copper, iron or other heavy metals or non-metallic ions comprising chloride, bicarbonate, protons, ammonium and substances that give rise to ammonium. The enzymes which are used may be a transferase, a hydrolase, an oxidoreductase or a lyase. An essential feature is a method to exclude interferences by ions by masking the interfering ions with a binding agent.