The effect of heat-treatment temperature and time on the mesophase formation process in the thermopolycondensation of isotropic petroleum pitch is studied by fluorescence and UV spectroscopy. The molecular structure of the basic components of isotropic petroleum pitch fractions is established. It is shown that UV spectroscopy is ineffective for analysis of multicomponent oil systems, whereas fluorescence spectroscopy allows study of the mechanism of molecular transformations in the mesophase petroleum pitch conversion process.