The title compound 4a, a purple transient whose main UV-vis absorption band occurs near 490 nm (epsilon = 5000 M-1 cm-1), has been generated by irradiation of 2,3,5,6-tetramethylene-7-oxonorbornane (5a). It has been observed directly by immobilization in frozen matrices or in polymer films and by time-resolved spectroscopy following nanosecond later flash photolysis of the ketone precursor 5a. Although the matrix-immobilized irradiated samples containing biradical 4a show a triplet electron spin resonance (ESR) spectrum at low temperature, some other species is the carrier, since the ESR spectrum persists even after complete photobleaching of the UV-vis spectrum characteristic of 4a. The singlet spin state for biradical 4a, predicted by theory, is confirmed not only by the absence of an ESR spectrum assignable to it but also by the appearance of a C-13 cross-polarization magic angle spinning NMR spectrum (delta = 113 ppm) observed when glassy preparations of 5a-2,3-di-(CH2)-C-13 are irradiated. The rates of dimerization of 4a in fluid media, (1.9-3) x 10(10) M-1 s-1, and the rate of capture by O2, 1.86 x 10(7) M-1 s-1, are also consistent with the behavior expected of a singlet biradical.