Samples of COSe and CSe2 were photolyzed at 77 °K, either pure or in matrices of Xe or trans-2-butene. Observed electron spin resonance spectra are assigned to Se(43P), to Se2, and to chains of selenium atoms, •Se(Se)nSe•. The selenium chains exhibit metallic behavior intermediate between that of sulfur and tellurium. It is suggested that polymeric chains of CSe are formed on warming photolyzed samples containing CSe2. The ultraviolet absorption spectrum of COSe vapor is reported.