A three-phase gas-liquid-solid system, where liquid is a continuous phase, was studied. This paper describes an investigation of the influence of gas and liquid velocity on the pressure drop through both an electroconductive and an inert bed formed by spherical glass particles (d=3 mm) in a rectangular column with a continuous liquid flow upward. The gas phase (hydrogen) is generated during water electrolysis on an electroconductive bed of silvered glass particles. On the basis of experimental data, the mathematical dependence of the relative pressure drop on gas and liquid velocity is determined. Furthermore, a mathematical model for determination of gas hold-up is proposed for the inert bed where the physical characteristics of the gas-liquid fluid are mimicked. (c) 2005 Elsevier B.V. All rights reserved.