Previous research on bubbling fluidized beds found that the voidage in the vicinity of an immersed surface is dependent on distance from the surface. Some authors have reported the presence of a thin gas gap between the surface and the emulsion phase. In the present study, experiments were performed to measure the thickness of the gas gap by a fiber optic probe measurement technique. The experiments, with Geldart C and A powders, were carried out with acoustic excitation to promote bubbling. The gas gap thickness was found to depend strongly on angular position around the surface of a horizontal tube and to vary slightly with sound pressure level and superficial air velocity. (C) 2004 Published by Elsevier B.V.