Oxo-hydroxy tautomerism and phototautomerism of 2-quinolinone, 1-isoquinolinone, 3-hydroxyisoquinoline, 2-quinoxalinone, and 4-quinazolinone were studied using the matrix-isolation technique. These compounds contain a benzene ring fused with a heterocyclic ring of 2-pyridinone, 2-pyrazinone, or 4-pyrimidinone. It turned out that direct attachment of a benzene ring to a heterocycle leads to a very pronounced increase of the relative stability of oxo tautomers (in comparison with the tautomerism of the parent compounds 2-pyridinone, 2-pyrazinone, and 4-pyrimidinone). The only exception concerns 3-hydroxyisoquinoline, where fusion with a benzene ring enforces rearrangement of the double- and single-bond system in the oxo tautomer. This destabilizes substantially the oxo form with respect to the hydroxy tautomer. The ratios of population of the oxo and hydroxy tautomers observed in Ar matrixes correspond to the tautomeric equilibria of the compounds in the gas phase. These equilibria were well reproduced by theoretical calculations carried out at the QCISD and QCISD(T) levels. The combined experimental and theoretical results reveal links between aromaticity and tautomerism. Moreover, a UV-induced phototautomeric reaction transforming the oxo forms into the hydroxy tautomers was observed for all (except 3-hydroxyisoquinoline) studied compounds. This photoeffect allowed separation of the IR spectra of the tautomers in question.