Spectrally resolved thermoluminescence of co-doped Dy:Tm:CaSO4 shows that the maximum peak temperature far the nominal 220 degrees C dosimetry peak differs by similar to 8 degrees C for Dy and Tm emission. This is interpreted as evidence that the rare earth ions farm part of a complex defect which variously provides both the charge trapping and, during heating, the radiative decay. The peaks have the same activation energies but different pre-exponential factors. Modifications of the material by thermal treatments using furnace or laser pulse heating convert the state of dispersion of the rare earth ions between isolated, pair or defect clusters, which alter the dosimetry efficiency. In some cases the modified geometries are detectable via movement of the emission lines. For rapidly quenched materials, discontinuities in the thermoluminescence responses are suggested to be indicative of new microcrystalline phases. Slow cooling degrades the efficiency but also indicates the presence of further thermoluminescence glow peaks within the region of the main dosimetry signal, Pulsed laser heating with a UV laser altered the glow curve and resulted in strong signals. Mechanisms for this process are considered.