The relationship of the electronic structure of oxazole, 2-phenyloxazole, and 2-phenylthiazole to the properties of the electronic excited states and transitions is examined. Spectral properties of these compounds in the free state (no effect from external perturbations) and in complexes with the proton and aprotic acids are measured and calculated by quantum-chemical PPP/S (π-approximation) and INDO/S (sp-basis) methods. Features of the electronic excitation of the atoms and the vibronic interaction of bonds in the singlet and triplet states are examined for a change of the various structural forms of azoles, which determine their spectral fluorescence properties and reactivity. Possible direction control of reactions and optimized syntheses of new compounds with given properties are discussed based on a study of the properties of the ground and excited states. ©1999 KluwerAcademic/Plenum Publishers.