The chemical nature of the important new chromogen ABTS [2,29-azino-di-(3-ethyl-benzthiazoline-6-sulphonic acid)] is described together with an account of the redox and spectroscopic properties of the system ABTS--H2O2--peroxidase. Keq. is calculated and a study of the steady-state kinetics over a whole range of substrate concentrations is reported. By using novel methods of kinetic analysis, an interpretation of the results is given which requires some extension of the classical peroxidase mechanism.
