The order-disorder transition of the CdMg₃ alloy was examined from the thermal point of view, by an apparatus of differential thermal analysis, of calorimetric type, with higher performance in respect of normal DTA apparatus, as to sensitivity, accuracy and resolution power. It was found that the transition: a) is practically complete in the range of temperature from 70° to 185°C, and b) occurs in two steps, corresponding to two peaks, the former being weaker at 100°-110°C, and the latter sharper at 154°C. Transition enthalpy is 5.8 cal g^-1, and entropy 0.65 cal/g atom °K, which indicates a degree of short-range order in the alloy above the transition. By a cooling rate through the transition as high as 4°C/min, the alloy practically reaches the highest possible order.