New data on the liquid residence time distribution for two-phase downflow of air-Newtonian and non-Newtonian liquids through packed beds of porous and non-porous particles are presented. The piston-dispersion-exchange model is used to describe the liquid flow. With porous particles the dynamic evolution of the tracer concentration in the particles is described in terms of diffusion phenomena. The axial dispersion is very important in the case of two-phase downflow of air-water (trickle flow regime) and air-CMC systems through fixed beds with porous particles, and is negligible in the case of non-porous particles. With the porous particles, a key value is the effective diffusion coefficient of the tracer in the pores of the particles.