Hydridocarbonyltris(trihenylphosphine)rhodium(I) dissolved in triphenylphosphine and capillary condensed in the pores of a support material, is applied in the heterogeneous hydroformylation of propylene at 90 °C and 1.57 MPa total pressure. The activity and selectivity of this new catalyst are high compared with those of known analogues. No sign of deactivation is observed over a period of more than 800 h. A small weight increase of the catalyst used is occasionally observed, and is attributed to some accumulation of low-volatile aldol condensation products in the pores. The aldol condensation reaction can be suppressed by using macroreticular polystyrene-divinylbenzene, XAD-2, or sodium-poor silica as support material. New bands in the IR spectrum of rhodium complexes are detected at 1947, 1993, 2002 and 2070 cm −1, which cannot be assigned to known rhodium complexes. Nitrogen capillary condensation proves the catalyst solution at 56% pore filling to be mainly located in the smallest pores of the support. X-ray microanalysis reveals a rather uniform distribution of the catalyst solution across a catalyst particle.