A analytical method to identify and quantitate residues of polychlorinated biphenyls, polychlorinated terphenyls, polychorinated naphtalenes and some chlorinated pesticides is described. These contaminants can be recovered from foods or other materials: a silicic acid coloumn chromatografic method previously developed is used to separate PCB's, PCT's, PCN's from organochlorinated pesticide residues. The PCB's, PCT's, PCN's residues identification and their subsequent quantitation can be achieved by conversion into well defined single perchloro-derivatives, viz decachlorobiphenyl, o.m.p. tetradecachloroterphenils and octachloronaftalene, detactable by GLC-EC. The reagnet of exhaustive chlorination of PCB's and PCT's residues is SbCl5, while the reagent used for the exhaustive chlorination of PCN's residues is SO2Cl2 with powdered anhydrous AlCl3 as a catalyst. The identity of perchloroderivatives obtained by chlorination of PCB's, PCT's, PCN's standard mixtures is confirmed on comparing their infrared spectra with the spectra of high purity standards.