A precision rotating-bomb calorimeter similar to that developed by Hubbard, Katz, and Waddington was constructed in this laboratory. It was calibrated by combustion of benzoic acid of purity 99.999 mole percent, which was prepared and its heat of combustion under standard bomb conditions was determined electrically in this laboratory. The precision of the results of the calibration experiments expressed in standard deviation of the mean was about two parts in ten thousand.