A series of experimental observations of the effects of applying electric fields to propagating reaction fronts in the iodate-arsenous acid system are presented. These demonstrate that changes in the local concentrations of reaction components due to ionic migration result in profound changes of selectivity of reactions within the front reflected in variations of front concentration profiles and velocities of propagation. The controlled switching on/off of the electric held leads to the formation of a variety of complex spatio-temporal concentration patterns. Results are obtained from a mathematical model having the generic features of the basic kinetic mechanism. These agree with the experimental observations and provide an explanation for variations in,reaction selectivity.