Particle concentration distribution images of a dense two-phase solid/air (plug) flow have been obtained at 10-ms intervals in a horizontal pipeline using capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) were decomposed to wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time-dominant particle distribution with a specific time frequency level can be visualized in cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high time frequency levels 6 and 7, is centrally located above the stationary layer. The distribution image at the front is reasonable when compared with the particle velocity distribution by conventional laser Doppler velocimetry (LDV) that indicates the discharging particles toward the front air phase at the centerline. (C) 2004 Elsevier B.V. All rights reserved.