Fixed bed reactors operated by feeding with two immiscible liquids are of increasing importance in chemical as well as biochemical reaction engineering. Based on biocatalysis studies, liquid/liquid/solid three-phase fixed bed reactors have been assessed with respect to the phase distribution in such reactors and the backmixing in each liquid phase. The systems were characterized by the presence of a continuous aqueous phase. The phase distribution in the reactor differed substantially from that in the feed stream, particularly when the feed had a low content of non-polar solvent. The accumulation of the non-polar phase in fixed beds of particulate solids was accompanied by blocking of the interstitial void volume by, and channelling of, the non-polar phase as well as enhanced backmixing of both the aqueous and the non-polar phase. Studies with various nonpolar solvents demonstrated that the choice of organic solvent is important for the operation of three-phase fixed bed reactors.