The hydrogen-bond interaction between formic acid and acetylene was studied by matrix isolation infrared spectroscopy coupled with ab initio computations. The l:l adduct of formic acid and acetylene was isolated in argon matrix, and the adduct formation was evidenced by the perturbation in the formic acid and acetylene vibrational modes. The structures, binding energies, and vibrational frequencies were calculated at the MP2 and B3LYP levels of theory using the 6-311++G(d,p) and aug-cc-pVTZ basis sets. Three minima corresponding to formic acid-acetylene complexes were identified at the highest level of theory (MP2/aug-cc-pVTZ). All of the three structures are cyclic with two hydrogen-bond interactions. In the global minimum complex A, the hydrogen atom in the carboxyl group of formic acid interacts with the pi-system of acetylene (O-H...pi). The second interaction is between one of the hydrogen atoms of acetylene and the carbonyl oxygen atom of formic acid. In the second complex B, the hydrogen bonding is a equivalent toC-H...O interaction involving a hydrogen atom of acetylene and the carbonyl oxygen atom of formic acid. In the second interaction, the formyl hydrogen atom of formic acid interacts with the pi-system of acetylene. In the third complex C, the main interaction involves one of the hydrogen atoms of acetylene and the hydroxyl oxygen atom of formic acid. In this complex, there is also a secondary interaction between the formyl hydrogen atom of formic acid and the pi-system of acetylene. This latter interaction is not reproduced using DFT calculations. The stabilization energy for the global minimum A is -4.9 kcal/mol, for the second complex B it is -2.9 kcal/mol, and that for the third complex C is -2.0 kcal/mol at the MP2/aug-cc-pVTZ level of calculation, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. The two most stable complexes A and B were identified in argon matrixes using FTIR spectroscopy. The calculated IR spectra of these complexes are in good agreement with the experimental spectra. Experiments with deuterated acetylene were performed to confirm the band assignments. The calculated frequency shift for the O-H stretching vibration of formic acid (-139 cm(-1) at MP2/aug-cc-pVTZ) is in excellent agreement with the experimental shift (-137 cm(-1)) for the O-H...pi complex A. This large shift in the O-H stretching vibration is a strong evidence for the O-H...pi interaction, which is consistent with the predicted large binding energy.