The conformational system of propionic acid (CH3CH2COOH) is studied in solid argon. It is predicted by the ab initio calculations that this molecule has four stable conformers. These four structures are denoted T-1, T-g(+/-), C-t and C-g(+/-), and they differ by the arran., gement around the C-O and C-alpha-C bonds. The ground-state T-t conformer is the only form present at 8 K after deposition of an argon matrix containing propionic acid. For the CH3CH2COOH and CH3CH2COOD isotopologues, narrow-band excitation of the first hydroxyl stretching overtone of the conformational ground state promotes the C-alpha-C and C-O internal rotations producing the T, and C, conformers, respectively. A subsequent vibrational excitation of the produced T-g(+/-) form induces 9 its conversion to the C-g(+/-) conformer by rotation around the C-O bond. In the dark, all of the produced conformers decay to the conformational ground state at different rates. The decay kinetics and its temperature dependence allow the identification of the conformers by IR absorption spectroscopy, which is supported by ab initio calculations of their vibrational spectra. For the CH3CH2COOD isotopologue, the excitation of molecules isolated in different matrix sites results in site-dependent photoisomerization rates for the C alpha-C and C-O internal rotations, which also confirm the identification of the photoproducts.