Summary: Further lines of the anomalous optical absorption spectrum in low metamict zircon have been observed in the infra-red. Low-temperature polarization studies have enabled a clear distinction to be made between lines due to the anomalous spectrum and those due to the normal spectrum of U⁴⁺ in zircon. Magnetic circular dichroism measurements showed, for the anomalous spectrum, only contributions due to the mixing of the energy levels by the magnetic field; thus the A spectrum results from singlet → singlet transitions. The anomalous spectrum appears to derive from U⁴⁺ incorporated in polycrystalline zirconia.