In this contribution we present the isomerizing hydroformylation of trans-4-octene to n-nonanal in a two-phase catalytic reaction system. Based on this two-phase system, the application of a thermomorphic multi-solvent catalytic reaction system will be presented which changes from a two-phase to a single-phase system by simply raising the temperature. This concept provides the possibility to overcome mass transport limitations which are typical problems in conventional two-phase reactions. (C) 2004 Elsevier B.V. All rights reserved.