Using an isoperibol twin-Dewar solution calorimeter, the enthalpies of reaction of normal lead styphnate monohydrate, monobasic lead styphnate, tribasic lead styphnate dihydrate, and pentabasic lead styphnate dihydrate were determined as -17.29, -61.92, -196.37 and -261.12 kJ mol(-1), respectively, which, with various ancillary data, gave the standard enthalpies of formation of -738.27, -981.70, -1995.99 and -2487.84 kJ mol(-1), respectively.