In an unstirred thin film of solution containing iodate and arsenous acid, a single propagating wave of chemical reactivity may be exhibited. The wave converts the solution from colorless to blue in reaction mixtures containing iodate in stoichiometric excess and starch indicator. The wave appears as a narrow blue band propagating through a colorless solution in reaction mixtures containing arsenous acid in stoichiometric excess. Waves were electrochemically initiated and propagation velocity was measured as a function of reactant concentrations. Iodide concentration was measured with an iodide selective microelectrode as a wave passed through the electrode position. The reaction-diffusion behavior is explained with a simple reaction mechanism autocatalytic in iodide. The coupling of chemical reaction with diffusion may give rise to propagating chemical waves, provided the reaction contains some appropriate form of kinetic feedback such as autocatalysis. Two fundamentally different types of reaction-diffusion waves have been observed in isothermal chemical systems: propagating pulses and propagating fronts.' The chemical waves in the excitable Belousov-Zhabotinsky2 (BZ) reaction are propagating pulses of reactivity. As a pulse passes through a point, the reaction intermediates undergo a concentration excursion which returns to its original state. In the BZ reaction, reactants are little depleted in an excursion and the pulse therefore effectively regenerates the kinetic state of the solution it passes thr~ugh.~ Propagating pulses have also been observed in the recently discovered oscillating iodate-chlorite- malonic acid rea~tion.~ Propagating fronts of chemical reactivity were observed in the iodate oxidation of arsenous acid over 25 years ago by Epik and Sh~b.~ In a recent study of this system,6 the wave behavior was confirmed and a qualitative explanation was proposed. In addition, an electrochemical method for initiating the waves in a thin film of solution was developed. As a front passes through a point the chemical species undergo a concentration excursion; however, instead of returning to their original concentrations, a new kinetic state is generated. A propagating front therefore converts a reaction mixture from one kinetic state to another. In the io- datearsenous acid reaction, the front converts the reaction mixture from an initial state where very little reaction has occurred to the state of thermodynamic equilibrium. Propagating fronts have also been studied in the bromate oxidation of ferroin.' Net reaction I describes the iodate-arsenous acid reaction when arsenous acid is in stoichiometric excess to iodate ((As(III)), > 3(10,-),). When iodate is in stoichiometric excess to arsenous IO3- + 3H3As03 = I- + 3H3As04