Chemical changes of phosphorous ligands in the rhodium complex catalyst system were studied using 31P NMR analysis. Triphenyl phosphite was readily oxidized by the small amount of oxygen contained in the synthesis gas to triphenyl phosphate in the course of hydroformylation. It is conceivable that this oxidation reaction is the cause of deactivation of the catalyst. The treatment of the synthesis gas with triisobutyl aluminum in toluene solution was effective in preventing the oxidation of triphenyl phosphite to triphenyl phosphate and in prolonging the activity of the catalyst for a long period. A 191-hr cotinuous run was made to establish catalyst activity, and 3, 220g of aldehyde was obtained using only 6.5mg of rhodium.