Trickle bed processes belong to the most widely used multiphase reactors and are applied on a large scale particularly for hydrotreatment of medium heavy and heavy oil fractions in the hydrocarbon processing industry. Laboratory scale reactors used in the development of new processes or in studies aimed at improving existing ones should therefore give accurate and meaningful test results which allow reliable upscaling. For reasons of efficiency in R&D, the size of the laboratory test units should be as small as possible without detracting from the meaningfulness of the data. The present paper discusses scaling rules of laboratory trickle bed reactors and the influence of scale parameters on residence time distribution and catalyst contacting under conditions that are typical for hydrotreatment of oils. The main limiting factors for downsizing of laboratory trickle-bed reactors are the distortion of the packing in the vicinity of the wall, axial dispersion and flow through the packing, and uneven catalyst irrigation. By dilution of a bed of catalyst particles with inert material of a much smaller particle size, it is possible to obtain kinetically representative data in very small laboratory reactors, viz., microflow reactors containing about 10 mL of catalyst.