Aqueous biphasic catalytical processes are still in the infancy of their significance : their decisive advantage (the easy separability of catalyst and product) will grow in importance even for reactions other than the economic and industrially proven hydroformylation. Despite relevance to industry, some basic facts about them remain unclear, such as the question as to where the reaction takes place. The location of the biphasic reaction is contested : is it the bulk of the aqueous catalyst solution or the interfacial layer aqueous/organic liquid? For RCH/RP＇s oxo process this question can be decided by methods of reaction modeling and the comparison with experimentally proven facts, thus leading to scale-up rules and appropriate kinetic models as a basis for the optimal reactor design.