The use of fine particles as a diluent of trickle bed reactor packings is a common method of ensuring good liquid distribution and catalyst wetting in laboratory scale tests, where the laboratory tests are operated at space velocities that correspond to industrial operation. In this paper, the effect of diluting the bed of industrial catalyst particles with fines upon the residence time distribution (RTD) of liquid flowing through the bed is investigated. It was determined that the dispersion coefficient decreased by up to 50% for the bed with fines, compared to that of an undiluted bed. The effect of using porous catalyst in the RTD experiment was also investigated, because many of the previously reported studies involved the case of a diluted bed of nonporous particles. The RTD for porous particles showed a more-pronounced tail, because of the holdup of liquid in the catalyst pores. A model with dynamic and static zones and intrapellet diffusion was fitted to the results, showing good agreement. This enabled the role of the static and dynamic zones, and effective diffusivity in a pellet upon RTD, to be investigated. The validated model may be used in predicting RTD in larger industrial-scale reactors.