A new solution calorimeter is described which combines the thermometric titration techniques with the precision and accuracy of conventional solution calorimetry. The calorimeter through use of a unique design has a low heat leakage,k= 1.1×10−3min−1, and a short equilibration period, 1–3 sec. Measurements of quantities of heat as small as 4 calories with an accuracy of 0.1% are possible. The heat of ionization of water at zero ionic strength has been determined to be 13.34±0.02 kcal/mole which is in excellent agreement with the value of 13.34±0.01 kcal/mole determined by conventional solution calorimetry. The large amount of information obtained from a single run makes this type of calorimeter a valuable addition to the field of solution calorimetry.