Semiempirical CNDO/2 oaloulations have been oarried out for isomerio xylenes and their protonated-complexes.The results indicate that the stabilities of these compounds are of the following order: looalised σ-complex >localised π-complex >Isomeric aylenes >CH₃ delocalised π-complex.Interpretations are given for the physical and ohemical properties of xylenes, such as isoinerio energy, ionization potential, acidity and basioity, proton affinity,eleotrophilic reaction, as well as the mechanism of isomerisation of m-xylene on acidic catalyst.