Absorption and fluorescence spectra for six new synthetic dyes of 4,8,4'-trimethylpsoralen and 4,4'-dimethylangelicin derivatives containing various terminal substituents at 5'-position have been investigated in different environments using a wide range of the DNA/ligand concentrations. Various spectral and binding characteristics of the DNA-ligand systems have been determined. General principles characterizing mechanisms responsible for changes in the fluorescent properties of nucleotide-specific dyes have been proposed; they take into consideration chemical structure of the dyes, properties of the environment, and degree of sorption on substrate.