The results from ethene hydroformylation at 173 degrees C showed that a Co(acac)(3)/SiO2 catalyst prepared from Co(acac)(3) precursor by gas-phase deposition was three times as active as a catalyst prepared by impregnation from cobalt nitrate, but ore-selectivities were similar The high propanal selectivities on the Co(acac)(3)/SiO2 seem to be related to the presence of highly dispersed active sites favouring CO insertion. As dispersion is decreased from 23 to 8% due to increasing metal content (from 5 to 16 wt%), ore-selectivity decreased from 39 to 25%. The activity of Co(acac)(3)/SiO2 remained unchanged during 68 h on stream. The gas-phase deposition technique described here is a promising method for the preparation of active, selective and stable heterogeneous hydroformylation catalysts.