The effects of ammonium salts as the additives in the [Rh(CH3COO)(2)](2)-Ph3PO catalytic system on the hydroformylation of mixed octenes have been studied. The yield of isononyl aldehydes increased when an ammonium salt such as (NH4)(6)Mo7O24. 4H(2)O. (NH4)2Cr(2)O(7). (NH4)(2)CrO4 or (NH4)(5)H-5 [H-2(WO4)(6)].H2O was added to [Rh(CH3COO)(2)](2)-Ph3PO-catalyzed hydroformylation of mixed octenes. The loss of rhodium in the distillation process for separating the products and the catalyst was reduced and the concentration of Rh in the distillate decreased from 0.115 to 0.057 ppm, when an ammonium salt such as (NH4)(2)CrO4 was added into [Rh(CH3COO)(2)](2)-Ph3PO-catalyzed hydroformylation. The stretching vibration (2040 cm(-1)) of the Rh-H bond of the active species was observed in the spectra of in situ IR. The investigation of in situ IR showed that the peak intensity of Rh-H vibration increased and the temperature range at which the Rh-H vibration peak could be observed in IR was widened when (NH4)(6)Mo7O24 4H(2)O was used as additive in the system [Rh(CH3COO)(2)](2)-Ph3PO.