Structural phase transitions influence the luminescence characteristics by modifying the spectra, sensitivity and luminescence efficiency in each phase. This has implications for dosimetric materials both for sensitivity and reproducibility. Previous work with dosemeter materials has not documented effects of different phases on thermoluminescence but Na2SO4 exists in several phases in the temperature range of interest for thermoluminescence dosimetry. Therefore spectrally resolved TL and radiothermoluminescence analyses of undoped and Dy3+ doped Na2SO4 have been performed. Dramatic differences between TL of slow cooled and quenched Na2SO4:Dy are explained in terms of the initial phase of the samples at room temperature. For undoped material the effect of phase conversions during heating shift the wavelengths of the emission bands. A model of Dy in small precipitate phases is considered.