The homogeneous catalytic hydroformylation of formaldehyde is an important reaction in the pursuit of using syngas as a building block for higher valued chemicals. In this study we investigated the mechanism of the rhodium-catalyzed, base-promoted hydroformylation of formaldehyde through the syntheses and characterization of model complexes which illustrated the structures and the relevant chemistry of the key intermediates in the catalytic cycle. In the study of the rhodium model species, the rhodium alkyl species, CH 3OCH 2Rh(CO) 2(PPh 3) 2, could not be intercepted because of a very rapid CO insertion into the rhodium alkyl bond. The resulting rhodium acyl complex, CH 3OCH 2C(O)Rh(CO) 2(PPh 3) 2, was stable enough to be characterized by FT-IR. When iridium complexes were used as models, all important catalytic intermediates including the anionic catalyst ([Ir(CO) 3PPh 3] −), the iridium alkyl species (CH 3OCH 2Ir(CO) 2(PPh 3) 2), and the iridium acyl species (CH 3OCH 2C(O)Ir(CO) 2(PPh 3) 2) were synthesized and unambiguously characterized. This model study clearly illustrated the novel anionic mechanism in the rhodium- or iridium-catalyzed, base-promoted hydroformylation of formaldehyde. The structural information obtained in this study is also important for the understanding of the mechanism of the hydroformylation reactions in general.