The use of a modified Thiele modulus, as a correlating parameter for partially wetted catalyst pellets, is compared with the exact solution of the diffusion and reaction problem. The study of a variety of partially wetted cylindrical catalyst pellets for first- and second-order kinetics indicates that the manner in which particles are externally wetted does not significantly influence the value of the effectiveness factor External and internal partial wetting of industrial-size porous extrudates is determined in a bench-scale hydrotreater at real hydroprocessing conditions by simulating the system response to a step change of feed using the sulfur compounds of a petroleum fraction as tracer molecules. To solve the corresponding mass balances in the interparticle and intraparticle domain, two different numerical methods are compared.