Measurements are reported of the heats of reaction at 25° of aqueous hydrazine with aqueous solutions of hydrogen chloride, hydrogen bromide, hydrogen iodide, liquid bromine, crystalline iodine, and crystalline samples of N-bromosuccinimide, and N-chlorosuccinimide. From these measurements, the following heats of formation were derived: ΔHf°[HBr, 53·5 H2O]=–28·75 ± 0·07 kcal./mole, ΔHf°[HI, 50·3 H2O]=–13·56 ± 0·03 kcal./mole, ΔHf°[N-bromosuccinimide, cryst.]=–80·35 ± 0·15 kcal./mole, ΔHf°[N-chlorosuccinimide, cryst.]=–85·58 ± 0·10 kcal./mole