A simple phenomenological theory is presented to describe the xenon poisoning of a molten fluoride fueled nuclear reactor which is sparged with a suitable inert gas. The extension of the theory to removal of other fission product species is indicated. The theory is illustrated by a parametric study. It is found that arbitrarily low levels of Xe135 can be achieved by adequate sparging of the fuel. Such sparging also frees these reactors from the trouble-some rise of the xenon poisoning during shutdowns. No reactivity transients of a serious nature are likely to result from changes in the rates of the xenon removal processes.