A surface perturbation method to determine reactivities is described which has application to the removal of reflectors, the lowering of water levels in reactors, the introduction of voided beam tubes, the insertion of black control rods, etc. A first-order approximation, using the unperturbed flux in the calculations, is shown to be in error for large perturbations. However, a simple one-energy expression is devised for the shape rather than the magnitude of the reactivity curve, that successfully predicts relative effects. The method is compared with an experimental determination of the reactivity worth of the variable upper reflector of the MITR.