The structure of the initial persistent radicals observed upon UV irradiation of poly(dialkylsilane)s (R2Si)n in solution, at any wavelength absorbed, is -SiR2-SiR-SiR2- as determined by EPR and ENDOR spectroscopy. A mechanism proposed for their formation consists of several steps, initiated by a new photochemical chain-breaking process, reductive elimination on a Si-Si-C unit, with the formation of trialkylsilyl terminal groups. The presence of such terminal groups in the irradiated product was established by GC-MS analysis after exhaustive irradiation at 254 nm. © 1988, American Chemical Society. All rights reserved.