The authors present the first experimental investigation of the residence time distribution of the liquid in a gas-liquid upflow fixed-bed reactor with porous particles. The PDE model (plug flow with axial dispersion and exchange with well-mixed stagnant zones) represents very well the experimental concentration profiles. The dynamic fraction of the liquid holdup obtained by fitting the PDE model to the experimental curves seems to indicate that only a relatively small fraction of the pore volume is accessible to the tracer. A critical evaluation of the results obtained and some possible further research directions are indicated. UWI'RODUCTION Catalytic reactors with cocurrent upflow of the gas and the liquid are used in processes where it is important to have high values of the liquid holdup, e.g. with highly exothermic reactions. One of the drawbacks of this kind of reactor compared with trickle-bed reactors is that the flow behaviour of the liquid is nonideal and that backmixing is considered to be more important than in trickle beds (Shah, 1989). During recent years, some authors have investigated the axial mixing of the liquid in this reactor type. The mixing of the liquid has been represented by using the following models: -Most authors used plug flow with axial dispersion (PD) (Stiegel and Shah, 1977a, b; Achwal and Stepanek, 1979). -Plug flow with exchange with. well-mixed stagnant zones was used by Sicardi et al. (1984). -One group (Skomorokov et al., 1986) used the PDE model (plug flow with exchange and axial dispersion).