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The effects of temperature and optical irradiation upon the intensity of the electron paramagnetic resonance of Chichibabin's hydrocarbon [4,4'-bis(diphenylmethyl)biphenyl] and related compounds have been studied. Relative intensities were determined by employing a field modulation amplitude sufficient to remove all structure and measuring first moments of the resultant derivative spectrum. It has been found that the room-temperature intensity can be maintained at low temperatures if the specimen is cooled rapidly enough. Further, the intensity can be enhanced by optical irradiation at -100°C. These effects are reversible, but a number of related irreversible effects also occur. The results suggest that the observable paramagnetic species in biradical solutions is a dimer or higher polymer of Chichibabin's hydrocarbon. As a result, the interaction between the unpaired electrons of the biradical is much smaller than would be expected for a monomer.