The hydrodynamics of a loop venturi reactor was studied using a down flow liquid jet ejector. The influence of the pressure, P, the superficial gas velocity, V-g,V-sup, and the presence of small activated carbon particles on the gas hold-up, epsilon(g), of aqueous systems in the main holding vessel was investigated. For the 2-phase system water-N-2 gas the influences of V-g,V-sup (in m s(-1)) and the gas density rho(g) (in kg m(-3)), on the observed epsilon(g) (in m(3) m(-3)) correlates as follows: epsilon(g) = 0.873 V-g,sup(0.55) rho(g)(0.17) where V-g,V-sup < 0.14 m s(-1) and 2.3 kg m(-3) &LE; ρ(g) &LE; 17 kg m(-3). The 3-phase experiments were performed with the system water-N-2 gas-activated carbon particles: Values of the solids fraction, ε(g), ranged up to 4.8 wt%. In these systems qualitatively similar results were observed regarding the influence of V-g,V-sup and P on ε(g). However, the effect of the fine carbon particles on the porosity is much more complex. The presence of a low solid content increases the gas hold-up, whilst a higher solid content reduces the porosity sharply. The positive influence of carbon fines on the porosity diminishes at higher gas pressures. Based on these results it is suggested that for the design of jet loop reactors for 3-phase aqueous systems with a low content of active carbon fines the effects of V-g,V-sup and ε(g) should be taken into account. © 2002 Elsevier Science Ltd. All rights reserved.