This paper describes the synthesis of n-nonanal by a consecutive isomerization-hydroformylation reaction of trans-4-octene using a homogeneous catalyst system consisting of Rh(acac)(CO)(2) and BIPHEPHOS. In the presence of syngas these two precursor molecules form in situ the catalytically active species. At a reaction temperature of 125 degreesC and a syngas pressure of 20 bar the linear C-9-aldehyde is formed in the solvent toluene in 88% yield and only few amounts of the saturated hydrocarbon and the branched C-9-aldehydes are obtained. In comparison to industrial applications the catalyst concentration has quite a high value of 0.5 mol% based on the olefin. With a reaction-time of 4 h and a selectivity of 89%, this catalytic system can be considered as highly reactive and highly selective. Performing the reaction in propylene carbonate instead of toluene rises the selectivity to n-nonanal from 89 up to 95%. This solvent has not only an influence on the selectivity but offers in addition the opportunity to recover the catalyst in a two-phase fluid-fluid system. In five recycling runs the catalytic phase could be reused without any loss of activity and selectivity to n-nonanal. The resulting total turnover number had a value of 866 while the turnover frequency was 34 h(-1). (C) 2003 Elsevier B.V. All rights reserved.