Thomsonite with ideal chemical composition and with an ordered framework structure was synthesised hydrothermally from zeolite Na-A, which was ground to X-ray amorphous, with 0.05 mol dm-3 CaCl2 solution at 200-degrees-C. The dehydration behaviour of the prepared thomsonite was examined by TG-DTA. It was revealed that thomsonite lost most of zeolitic water below 450-degrees-C in three steps at about 180-degrees, 340-degrees- and 390-degrees-C. The peak profiles of the two higher-temperature endotherms were sharp and similar, and the weight loss at each step was approximately equal.