The liquid residence time distribution has been evaluated in a trickle bed reactor applying maximum liquid and gas velocities of respectively 10 and 140 mm . s(-1). The influence of the liquid viscosity has been studied, using the piston now with axial dispersion and mass exchange (PDE) model to evaluate the experiments. The residence time of the liquid phase could be well correlated to the liquid Reynolds and a modified Galileo numbers. The fraction stagnant liquid holdup as determined using the PDE model;is less than half the value found from draining experiments and stays constant with increasing liquid velocity for water, while it increases for the more viscous liquid. The Bodenstein number in the dynamic zone coincides with single phase flow. For water the NTU between the dynamic and the stagnant zone is about 0.4, while for the viscous Liquid it increases gradually to 2.5 with increasing liquid velocity.