Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans-->cis reaction is promoted using resonant excitation of a number of modes in the 3500-7000 cm(-1) region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials: For excitation of acetic acid at energies above the predicted isomerization energy barrier (greater than or equal to4400 cm(-1)), the measured quantum yields are in average 2%-3%, and this is one order of magnitude smaller than the corresponding:values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans-->cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis-->trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans-->cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans-->cis reaction quantum, yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis-->trans dark reaction, the observed temperature and medium 'effects' indicate the participation of the lattice phonons in the tunneling-induced process. (C) 2004 American Institute of Physics.