CCSD and EOM-CCSD calculations with the aug＇-cc-pVDZ basis set have been carried out to investigate the structures, energies, and vibrational spectra of H2CO, protonated H2CO, and hydrogen-bonded HF:H2CO complexes in ground and excited (1)n-->pi* states. The computed structures, vibrational spectra, and excitation energies of H2CO are in agreement with experimental data. In the (1)n-->pi* state two protonated isomers, H2COH+ and H3CO+, exist. These differ in stability by 14 kcal/mol, with H2COH+ being more stable. The proton affinity of excited H2CO is 17 kcal/mol less than the ground-state proton affinity. Two hydrogen-bonded complexes HF:H2CO also exist in the excited (1)n-->pi* state. The more stable complex has hydrogen bond formation occurring at the oxygen and is approximately 2 kcal/mol less stable than the corresponding ground-state complex. The complex with hydrogen bonding at the carbon is only a weakly bound, Born-Oppenheimer minimum.