Using an isoperibol twin-dewar solution calorimeter, the enthalpies of reaction of thallous styphnate and the half-acid salt of thallous styphnate were determined as +64.56 and +26.80 kJ mol-1, respectively, which with various ancillary data give the standard enthalpies of formation of -527.56 and -492.05 kJ mol-1, respectively.