This study investigated the photochemical behavior of the ferroin-bromate-benzoquinone reaction in a batch system, in which the reduction of ferriin was dominated by bromide ions rather than by organic substrates as known in most of bromate oscillators. The chemical oscillations exhibited ultrasensitive response to illumination, where both the oscillation frequency and lifetime were significantly increased by ceiling light with an intensity of less than 20 mu W/cm(2). As the light intensity was increased, the influence of light evolved from constructive to inhibitory. At a constant illumination, the influence of light also underwent a transition from constructive to inhibitory when the acid concentration was increased from 0.03 to 0.09 M. NMR measurements and absorption spectroscopy indicated photoenhanced bromination of benzoquinone-and photoenhanced reduction of ferriin by bromide ions.