Residence time distribution (RTD) experiments were carried out in pilot plant reactors using diluted and non-diluted beds. Feeds with properties at ambient conditions similar to the properties of hydrotreating feeds at reaction conditions were used. The liquid flow non-idealities were studied with the use of the axial dispersion model. The behaviour of diluted and non-diluted beds was investigated. The effect of dilution and the gas and liquid feed velocities on the liquid hold-up and Pe is presented.