The composition of Chlorofen, a technical formulation of polychlorinated biphenyls (PCBs) produced in Poland, has been investigated using GC-ECD and GC-MS techniques. Chlorofen was a highly chlorinated mixture (63.6% Cl), mainly composed of PCB members with 5 to 9 chlorine atoms which comprised 99.55% of total PCBs. The average number of Cl per molecule in Chlorofen was 7.3, and the average molecular weight was 405.4. It was found that Chlorofen contained at least 59 PCB congeners with the major components such as IUPAC No. 153 of hexa-, 176, 180, 187 of hepta-, 194, 195, 198, 201/196 of octa- and 206 of nonachlorobiphenyls. Additionally, three components of highly toxic non-orthochlorine substituted coplanar PCBs (IUPAC Nos 77, 126 and 169) were also quantified at a concentration of 0.52, 0.25 and 0.43 μg g−1, respectively, the latter (169), showing a greater content than in Aroclor and Kanechlor formulations.