Olefins, such as allyl alcohol, propylene, 1-hexene, styrene, methyl acrylate and acrolein, were hydroformylated with formaldehyde to form corresponding aldehydes. Allyl alcohol and methyl acrylate which have oxygen at b position to double bond showed higher reactivities and selectivities to n-products than those of olefins without oxygen such as propylene, 1-hexene and styrene. 4-Hydroxybutanal was formed as a major product from the reaction of allyl alcohol with ลฝ . formaldehyde under nitrogen atmosphere at 1008C and 5 h yield: 26%, n-riso-s21 . The addition of syn-gas or excess phosphine to the reaction mixture inhibited hydroformylation with decreasing n-riso-ratio. The hydroformylation of olefins without oxygen gave better yield to aldehydes under syn-gas condition. From in situ IR experiments, the formation of formyl ligand coordinated to metal center was observed. Variable temperature NMR experiment was also performed to verify the mechanism. Based on those observations, a plausible reaction mechanism was proposed.