The bound water content of original and frozen activated sludge and an inorganic sludge were measured via differential scanning calorimetry (DSC) and drying tests. Original sludge contained a significant level of bound water which was reduced by about 50% after freeze/thaw treatment. A simple thermodynamic interpretation showed that liquid-solid binding strength played an important role in depression of the freezing point of bound water in a sludge. The choice of a threshold temperature in a DSC test was equivalent to definition of a threshold binding strength dividing bound/non-bound water content. The possible close relationship between the bound water content measured via DSC and the drying test is discussed. The importance for sample uniformity was also examined.