Formic acid molecules are trapped in two predominant local environments (sites) when isolated in an argon matrix at 8 K. Using narrowband tunable infrared (IR) radiation, we performed site-selective excitation of various vibrational modes of the lower-energy trans conformer. For all excited modes, ranging from 7000 to 2950 cm-1, we detected site-selective isomerization to the higher-energy cis form. By measuring the IR absorption of a selected band of the cis conformer as a function of the excitation frequency, the reactive vibrational excitation (RVE) spectra were obtained. The trans-->cis isomerization quantum yields for the excited modes were determined. Remarkably, very high absolute values were obtained for the quantum yield (up to 40%) at excitation energies above the reaction barrier. The efficiency of the photoinduced isomerization is essentially independent of the excited vibrational mode in a broad energy interval. Even when the excitation energy was below the reaction barrier, IR-induced rotational isomerization was observed, which indicates tunneling from the vibrationally excited trans conformer to the cis form. Using the RVE spectra, phonon sidebands were detected on the high-frequency side of the zero-phonon-line of the OH stretching mode of trans-formic acid. These weak and broad bands were not observed in the absorption spectra. Additionally, a relatively narrow band blueshifted by 6 cm-1 from the OH stretching fundamental mode was assigned to a librational satellite based on simulations using the hindered rotation model for an asymmetric top trapped in an octahedral crystal field. (C) 2003 American Institute of Physics.