The molecular complex formed between the formaldehyde molecule and the lithium atom was studied bp ab initio molecular orbital calculations. Electron correlation energies were calculated by both the Moller-Plesset perturbation and multireference configurational interaction (MRCI) methods. Three local minima were located in the electronic ground state of the complex at both the MP2/6-31+g(2d,p) and MRCI(4s3p2d/3s2p)levels. Two of the local minima are ion pair in nature (Li+CH2O-), and the third one has the character of the alkali metal atom-Lewis base bonding (Li...CH2O). The latter is designated as a complexation pair state. The bond dissociation energies are 20.70 and 19.14 kcal/mol for the two ion pair states and 7.73 kcal/mol for the complexation pair state, respectively, at the MP4/6-311+g(2d,p) level. Consistent results were obtained with the MRCI approach. The energy barriers between the ion pair states and the complexation pair; state were searched at MRCI level, and the transition states were determined. The general features of the adiabatic potential surface of the electronic ground state of the complex were described. The effects of complexation on the geometry, vibrational frequencies, and the electronic structure of formaldehyde were also discussed. For completeness, the positively charged ion of the complex was also studied, and the corresponding ionization potential of the neutral states were calculated.