CaSO4:Tm and CaSO4:Dy were found to be free from the serious disadvantage of rapid fading in CaSO4:Mn and yet retain the outstanding property of high sensitivity of CaSO4:Mn, the most sensitive thermoluminescence dosimeter available at present. Glow curves of both CaSO4:Dy and CaSO4:Tm are rather simple showing a main peak at 220°C and shoulders at 80°C and 120°C. Emission spectrum of CaSO4:Tm consists of a main sharp peak at 450 mμ with minor ones at 360, 470 and 520 mμ and CaSO4:Dy has two main peaks at 480 and 570 mμ without any minor one. The dosimetric characteristics of CaSO4:Tm and CaSO4:Dy are excellent except for their non-tissue equivalence: 1. Little fading (1–2% per month). 2. Minimal detectable exposure is lower than 200 μR with standard deviation of ±20%, by a commercial readout instrument. 3. Dose response of CaSO4:Dy is linear up to 3000 R and supralinear over 3000 R, and that of CaSO4:Tm is linear up to 300 R and supralinear over 300 R. 4. Energy dependence is large for photon energies below 200 keV but a compensation shield made of tin and lead alters the response until it is within 40% down to 30 keV. 5. Tribothermoluminescence is very low (about Sloth of that in LiF). 6. The thermoluminescence efficiency remains constant for more than 100 readout cycles sufficient to erase the previous dose.