[en] The combination of photoluminescence spectroscopy with the radioactive isotopes ⁷Be, ⁷¹As, ¹¹¹Ag, ¹¹¹In, ¹⁹¹Pt, ¹⁹³Au and ¹⁹⁷Hg is shown to provide definitive proof of the chemical identity of impurities producing photoluminescence spectra in all classes of semiconductors. The isotope ⁷¹As is used to show that radioactive isotopes can provide a powerful means of producing and studying a fundamental crystal defect such as an anti-site. Factors governing the luminescence intensities which can lead to apparently anomalous results are also discussed