The local bubble frequency, the local phase holdups and the local bed porosity in three-phase fluidized beds were measured. Air, water, and 250 μm glass beads were used as the gas, liquid, and solid phases. The effects of gas and liquid velocities on phase holdups were studied. Time-averaged local gas holdup and bubble frequency were measured by an optical fiber probe. Mass balances were performed at various axial positions along the fluidized beds to check the accuracy of the measurements.