The Mixture, infrared spectrum, arid.photo-chemistry of 5-hydroxyquitoline (5HQ) were studied by matrix isolation infrared spectroscopy, complemented by theoretical calculations performed, at the DFT(B3LYP)/6-311++G(d,p) level of approximation. According to the calculations, the trims conformer Of 5HQ (with the OH 'group pointing to the opposite direction of the pyridine ring of the molecule) is more stable than the cis form (by similar to 88 kJ mol(-1)). The main factors determining the relative stability of the 'two conformers were rationalized through natural bond orbital (NBO) and charge density analyses. The compound was trapped in solid nitrogen at 10 K, and its infrared spectra registered and interpreted; showing the sole presence in the matrix of the more stable trans conformer. Broadband in situ UV irr-adiations (lambda >= 288 nm and lambda >= 235 nm) allowed for the observation of different chemical transformations, which started by excitation to the S-1 state of 5HQ followed by homolytic cleavage of the O-H bond, and subsequent reattachment of the H atom to the 5HQ radical to form quinolin-5(6H)-one and quinolin,5(8H)-one. The first of these two quinolinones was found to convert to open-ring isomeric 'ketenes, especially when irradiation was performed at higher,energy, whereas the second is rather stable under the used.experimental conditions. As a whole, the observed photochemistry of matrix-isolated 511Q-closely matches those previously reported for phenol and thiophenol. A detailed mechanistic interpretation for the observed photochemical processes is here proposed, which received support from time-dependent DFT calculations.