A modified, off-the-shelf, gamma-radiation density gauge has been interfaced into a microprocessor and mounted to a movable assembly mechanism which allows fluctuating bed density to be monitored on-line at any axial height in a fluidized bed. The signal processing required to determine expanded bed height L f , dense phase voidage ε D , dense phase superficial gas velocity u Do , cross-sectional centerline bubble phase volume fraction 〈δ B 〉 t c , bubble size d b , frequency f b , is described in detail. Example measurements are presented as made on both a cast-acrylic model column and an industrial, pilot-scale, high-pressure, fluidized-bed reactor. Accuracy of the measurements is discussed.