Photoinduced transformations of 2-styrylquinazolinones in solutions were studied using absorption and NMR spectroscopy methods. A possibility of control of the photochemical isomerization rate of quinazolinone 2-(hydroxyaryl)ethenyl derivatives by changing the pH of the medium was demonstrated. The bases and the solvent nature also affect the luminescence intensity of solutions of these compounds in the wavelength range of 550-650 nm. The differences in the steric organization of the ortho-hydroxystyryldiazinone system in crystals and in solutions related to the turn of the aryl group were found. Their influence on the competing processes of luminescence and photochemical transformation of the ethylene fragment were shown. The fact of reversible photo/thermal E-Z-isomerization was established for (E)-2-(2-hydroxystyryl)-3-phenylquinazolin-4(3H)-one.