The hydroformylation of hexene-1 catalyzed by Rh_4 (CO)_12 and 〔Rh(CO)_2Cl〕_2 andthe effects of solvents (benzene, n-heptane and THF) and PPh_3 on the activity andselectivity of the catalysts were studied.The results indicated that the activity of Rh_4(CO)_(12) decreased with increasing solventpolarity and it did not change appreciably with addition of PPh_3 when the molar ratioPPh_3/Rh_4(CO)_(12)≤4. Thc activity of Rh_4(CO)_(12) was over twice as large as that of〔Rh(CO)_2Cl〕_2 when calculated according to the number of moles of rhodium clusters inbenzene without PPh_3, but the activities of both Rh_4(CO)_(12) and 〔Rh(CO)_2Cl〕_2 wereabout the same when calculated according to the number of rhodium atoms. It is suggestedthat the clusters probably dissociated into the similar active rhodium mononuclear speciesunder the reaction conditions and the species formed a cluster similar to Rh_4(CO)_(12) againwhen the conditions were reduced to the ambient temperature and pressure. The hydroformylation of hexene-1 catalyzed by Rh_4 (CO) _12 and [Rh (CO) _2Cl] _2 and the effects of solvents (benzene, n-heptane and THF) and PPh_3 on the activity and the aselectivity of the catalysts were studied.The results indicated that the activity of Rh_4 (CO) _ (12) decreased with increasing solvent polarity and it did not change appreciably with addition of PPh_3 when the molar ratio PPh_3 / Rh_4 (CO) _ (12) ≤ 4. Thc activity of Rh_4 (CO) _ (12) was over twice as large as that of [Rh (CO) _2Cl] _2 when calculated according to the number of moles of rhodium clusters in benzene without PPh_3, but the activities of both Rh_4 (CO) _ (12) and [Rh (CO) _2Cl] _2 wereabout the same when calculated according to the number of rhodium atoms. It is suggested that the clusters probably dissociated into the similar active rhodium mononuclear species under the reaction conditions and the species formed a cluster similar to Rh_4 (CO) _ ( 12) againwhen the conditions were reduced to the ambient temperature and pressure.