Residence time distribution (R.T.D) experiments were carried out in a pilot scale upflow reactor. Feeds with properties similar to the properties of feeds at reaction conditions were used. The Piston-Dispersion model is used to decribe the liquid flow non-idealities. The effect of velocity and particle dimensions on the Peclet number and liquid holdup are discussed. Predicted deviations of the performance of a typical pilot hydrotreater From that of an ideal plug flow reactor are presented.