High-speed images of bubbly flow coupled with simultaneous passive acoustic measurements were carried out in a bubble column staged with structured fibrous catalysts. Local observations made with a high-speed camera system allow a detailed understanding of the influence of fabric structure on bubble formation processes and the determination of the Sauter bubble diameter. With acoustic measurements, the building frequency of bubble swarm formation is identified to be a linear function of the superficial gas velocity. Due to bubble size and building frequency of bubble swarm the percentage of simultaneously operative sites (holes at which bubbles were formed simultaneously) is calculated. (C) 2003 Elsevier Science B.V. All rights reserved.