The two-phase hydroformylation of lower olefins (Ruhrchemie-Rhone/Poulenc process) is a well known example of a multiphase fluid-fluid reaction (two liquid phases and a gas phase). Typically, reactions of this type are performed in stirred tank reactors. Investigation of the reaction in a packed tubular reactor revealed, that under very unconventional operation conditions space-time yields of 1-3 t m(-3)h(-1) are accessible. Compared to the conventional process, this is an increase by a factor of 10. This very surprising effect was clarified by simultaneous modelling of mass transfer and chemical reaction. Numerical simulations show that the effects are not due to a simple increase in mass transfer area. Alternative explanations are discussed. Further investigations in a mini pilot plant reveal a very simple yet highly efficient process design. Moreover, and in contrast to the conventional process higher olefins up to 1-octene can be converted to the respective aldehydes at reasonable space-time yields and with high selectivity. Successful application of the process principles to other fluid-fluid reactions suggests. that we have possibly found a more general concept for multiphase reactions. (C) 2003 Elsevier Science B.V. All rights reserved.