The heat of reaction of solid tris(hydroxymethyl)aminomethane (THAM) with 0.1 M hydrochloric acid solution has been determined calorimetrically at 10, 25, and 40°C. The experimental conditions suggested by the Standards Committee of the U.S. Calorimetry Conference were followed. Values of Δ H° of −7.644±0.014, −7.104± 0.008, and −6.555±0.012 kcal/mole at 10, 25, and 40°C, respectively, are reported. Results of the study are compared with those of other workers, and THAM is recommended for use as an interlaboratory comparison standard for solution calorimetry.