Phenyl and 4-methylphenyl isocyanide dichlorides are models for byproduct that may be formed in the later stages of certain polyurethane production chains. Photochemical electron paramagnetic resonance (EPR) studies (lambda > 310 nm), using the spin trap, N-tert-butyl-alpha-phenylnitrone, confirm a previously made suggestion that ArN=CCl2 can behave as a chlorine radical source. EPR spectra recorded during and after irradiation and supported by simulations evolve over time and indicate formation of the short-lived spin trap-Cl-center dot adduct and a longer lived benzoyl-N-tert-butylnitroxide radical. Photolysis of C6H5N=CCl2, either alone or mixed with methylene diaryl isocyanate species, in o-C6H4Cl2, a polyurethane process solvent, led to the formation of mixtures containing dichloro- and trichlorobiphenyl isomers.