The electronic structure of some monomethyl derivatives of alloxazine has been investigated by the INDO/S-Cl-1 method in both the ground and in the two first-excited singlet states. A description of the intrinsic states of molecules by the extensive use of HOMO/LUMO coefficients is suggested. The proposed method both quantitatively and qualitatively characterises satisfactorily various phenomena arising from spectral properties, from reactivity to the interpretation of NMR spectra. The proposed method shows explicitly non-accidental conjunctions, arranging the terms in certain, logical sequences between properties of a molecule and of a substituent in its definite site.The significance of these results is reviewed in relation to the experimentally identified chemical and spectral properties. The influence of substitution by a methyl group on the electronic charge distribution and on the variation of dipole moment, transition moment, orbital energies and electronic transitions are examined. The importance of the frontier parameters of the HOMO/LUMO state in a description of substitution effect is emphasized.