Pilot-scale hydrotreatment reactors often operate with gas-liquid upflow. For this type of operation there is a lack of information concerning the nonideal characteristics of the liquid flow, especially at elevated pressures. In this study, the Peclet number and liquid holdup of a pilot-scale upflow reactor were derived by residence time distribution experiments. The experiments were carried out at various operating pressures with different feed gases. The liquid-phase mixing was found to be identical for constant feed volumetric superficial velocities calculated at the system pressure. The liquid holdup diminished when elevated pressures were used. The use of very high gas flow rates during the shift of the operating pressure can intensify the liquid-phase nonidealities.