A gamma-ray transmission gauge was applied for the measurement of the interzone-layer parameters in an extraction tower. The “resolving power” function of the gauge was used to obtain the optimum design parameters, such as gamma-ray energy, source-to-detector distance, and gauge location on the separation tower. Results from a prototype gauge operated on a furfural extraction tower in the lubrication oil industry are presented and discussed. The mathematical model described here offers guidelines for analyzing similar gauging problems.