Previous work on the catalytic decomposition of sodium hypochlorite streams has focussed on high levels of destruction (typically >99.99%), with exit concentrations typically in the 1-50 ppm range. This design requires low space velocities and minimisation of forward mixing, and a multi-bed downflow reactor has been successfully utilised in industry. This paper reports on the development of an alternate reactor for lower conversion levels-in the order of 90%. At the space velocities required to achieve this economically, the downflow bed becomes infeasible due to classic hydraulic limitations. The scenario hare differs from those reported in the literature for upflow packed bubble columns; the gas is evolved through the height of the reactor, and the gas flux therefore varies over the height of the catalyst bed, with implications for gas phase hold-up. The gas phase hold-up is expected to exert considerable influence on reactor performance as the as will occupy space thereby reducing the residence time of the liquid phase and by blinding the catalyst surface. The study was based on experimental work in the laboratory, and later on a 0.2 m diameter pilot. In analysing the data, kinetic effects were accounted by the use of intrinsic rate constants from previous work. This allowed decoupling of the kinetic effects from the hydrodynamic effects. The results indicated a strong dependence of the apparent catalyst performance on the liquid and gas superficial velocities, which have been observed in the literature to be the key variables affecting gas phase hold-up in packed bubble columns. The performance data measured on the pilot unit mapped well onto the laboratory data, indicating the process scales simply. (C) 2003 Elsevier Science B.V. All rights reserved.