Raman spectroscopy combined with matrix isolation technique was employed to study formic acid monomer and its dimers. Nine fundamentals of the HCOOH monomer were identified. Additionally, the OH torsion overtone is observed and it is engaged in Fermi resonance with the delta COH mode. Several bands of the cyclic (C-2h) dimer were identified and assigned. The less stable open form (C-s) of the dimer was also present in the experiment. Three intermolecular modes for the cyclic and one mode for the open dimer were found in the low frequency region. The experimental studies were supported by harmonic and anharmonic DFT calculations. (C) 2008 Elsevier B. V. All rights reserved.