The gas-phase FT-IR spectrum of the formic acid-trifluoroacetic acid (FA-Tfa) hydrogen-bonded complex, or bimolecule, was obtained by numerical analysis of the FT-IR spectrum of a mixture of FA and Tfa vapors. Nineteen out of 24 vibrational modes predicted by ab initio frequency calculations to occur in the mid-IR range (400-4000 cm(-1)) were observed as well-defined absorbance peaks, with the other five occurring as complex or overlapped regions. Several hydrogen-bond-influenced vibrations of each monomer were identified, including C=O stretching and COH in-plane and OH out-of-plane bending. These occurred at 1701, 1403, and 871 cm(-1), respectively, for FA, and at 1774, 1325, and 942 cm(-1), respectively, for Tfa. The hydrogen bond donated by Tfa in the bimolecule appears to be stronger than that in the Tfa dimer, while the FA-donated hydrogen bond is weaker than that in the FA dimer. Geometry optimization and vibrational frequency calculations were carried out at 21 levels of theory up to B3LYP/aug-cc-PVDZ. All levels of theory predicted an unsymmetrical complex, with hydrogen bond distances of 1.608 Angstrom and 1.706 Angstrom donated by Tfa and FA, respectively, and the corresponding 0(...)0 distances of 2.620 Angstrom and 2.704 Angstrom (B3LYP/aug-cc-PVDZ). These values differ from the symmetrical, or nearly symmetrical, structure derived from microwave spectra (Costain and Srivastava, J. Chem. Phys. 1964, 41, 1620-1627; Martinache et al. Chem. Phys. 1990, 148, 129-140). In agreement with previous experimental findings, the bimolecule was predicted to be more stable than either homodimer, with a calculated DeltaH(complexation) of -14.2 kcal/mol (B3LYP/aug-cc-PVDZ). Mulliken population analysis predicted a polar complex with a transfer of 0.02-0.03 protons from Tfa to FA and a predicted dipole moment of 2.3-2.4 D, depending on the level of theory. The combined spectroscopic and computational evidence indicates that in this complex the Tfa-donated hydrogen bond is strengthened more than the FA-donated hydrogen bond is weakened.