Established techniques for measuring thermal reactor flux and power which utilize, respectively, arbitrary placement of foils and overall heat balance, have limited accuracy and are difficult to apply. The proposed method is based on 135Xe accumulation, as manifested by control-rod positions, plus a set of relative flux factors taken over the entire core instead of only at isolated positions. The technique involves only routine operating data, and required calculations are quite manageable by machine computation. After development of the theory, application is illustrated using a set of data from the Union Carbide Nuclear Company research reactor at Tuxedo, New York. Results are quite reliable when rod positions during the shutdown period are used, but operating period ratios introduce large inaccuracies by magnification of routine imprecision in operating data. It is concluded that the method offers an easily obtainable check on the usual heat-balance information for heterogeneous reactors, particularly if applied to shutdown data or to a xenon equilibrium run.