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We report a new photoluminescence process in epitaxial Sil-,GeX layers grown on Si by rapid thermal chemical vapor deposition which we attribute to the recombination of excitons localized at random alloy fluctuations. This luminescence is characterized by saturation at very low excitation densities (N 100 PW cm-*), very long decay times (> 1 ms), and high quantum efficiency at low excitation. We have directly measured an external photoluminescence quantum efficiency of 11.5 *2%.