The spatial flux oscillations that may occur in a power reactor as a result of xenon and local temperature effects have been studied on a general-purpose electrical analog computer. The linearized forms of the two-group diffusion equations with xenon-dependent coefficients are solved in one dimension using finite space intervals. The xenon-dependent coefficients are obtained at each space point by solving the linearized forms of the iodine and xenon equations using continuous integration, one second of computer time representing one hour of reactor time. Thus at each space point four operational amplifiers are required—one each for iodine, xenon, fast flux, and slow flux. The present application has ten space points on a radius, or on the half-thickness of a slab, requiring 40 amplifiers and 80 potentiometers. Good agreement is obtained with modal theory for predictions of the threshold fluxes in simple cases. Unlike some applications of modal theory, it is not assumed in the case of a persisting or pure mode that each of the oscillating variables is the product of a real function of space and a function of time. In fact it is found that the space shape changes continually during a cycle of an infinite train of oscillations, this behavior repeating in every cycle. This is partly a result of the xenon's lifetime against burnup varying through the reactor. The change of shape is less marked for the flux and iodine than for the xenon, and is most marked in the case of high equilibrium flux. At a central flux of 2.1 × 1014 cm−2 sec−1, the maxima in the xenon occur 2.5 hr later at the outside of a cylinder or of a slab than at the center. Some examples of two-group mode shapes are also given for reflected and flattened reactors.