The occurrence of spatial domains of large amplitude oscillation on a background of small amplitude oscillation in a reaction-diffusion system is called localization. We study, analytically and numerically, the mechanism of localization in a model of the Belousov-Zhabotinsky reaction subject to global feedback. This behavior is found to arise from the canard phenomenon, in which a limit cycle suddenly undergoes a significant change in amplitude as a bifurcation parameter, in this case the

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... ck strength, is varied. In the system studied here, the oscillations arise via a supercritical Hopf bifurcation, but our analysis suggests that the same mechanism is relevant for systems undergoing a subcritical Hopf bifurcation.