In ethene hydroformylation over Rh/active carbon prepared from RhCl3, 3-pentanone was formed in addition to propanal, which was a common product in the reaction. In the case of propene hydroformylation, however, butanal was exclusively formed and no appreciable formation of ketones was observed. On the catalyst treated with CO at 473 K, both activities for propanal and butanal formation were observed to be higher than those on the catalyst untreated. An approximate linear relation between the activity for propanal formation and that for butanal formation was observed, indicating that the active sites for aldehyde formation can catalyze the reaction for both ethene and propene. On the other hand, the activity for 3-pentanone formation was reduced by the CO treatment. These results indicate that the transformation of active species for 3-pentanone formation into the active species for aldehyde formation was induced by the CO treatment, On the catalyst treated with CO, a value for n/iso-ratio in butanal formed was observed to be higher than that on the catalyst untreated. These results suggest the formation of new active Rh particles in the narrower pores, On the basis of the results obtained, the active species for 3-pentanone formation was thought to be located in the narrow pores, where the formation of reaction intermediates of C-7-ketones from propene would be strongly suppressed because of their bulky structure compared with that for 3-pentanone.