CaSO₄:Dy and CaSO₄:Tm are highly sensitive phosphors used in the thermoluminescence (TL) dosimetry of the ionizing radiations. It is shown that, by using P, Mo or SO₃ as co-dopants, the TL sensitivities can be increased by 40-100%. The increase in sensitivity originates in various processes, which are discussed. P acts as a charge compensator, allowing the incorporation of Dy in higher concentration without causing quenching. Mo increases the efficiency of trap filling. It is claimed that use of Mo as a co-dopant will improve the sensitivities without disturbing the other dosimetric characteristics.