A decomposition treatment of density of states in combination with PM3 molecular orbital calculations was used to reveal the fingerprints of electronic structures of two prototypical electroluminescent molecules, tris(8-hydroxy-quinoline)aluminum (Alq(3)) and N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB). High convenience and accuracy of such a treatment were found for these large organic molecules in the determinations of (1) the distribution of important molecular orbitals such as the highest occupied orbital and the lowest unoccupied orbital; (2) the contribution to valence and conduction bands as well as gap states from constituent atoms, and thus the attribution of ultraviolet photoemission spectrum; (3) the sites and properties of reaction and excitation of a molecule; and (4) the localization property of electronic states. In particular, this study indicates that Alq(3) is most possibly attacked by other atoms at the oxygen atoms while the reaction site for NPB is at the nitrogen atom.