On the basis of the thermodynamic hypothesis and using the thermodynamic approach, a binding strength was calculated to define the physical meaning of bound water content of sludge as measured by dilatometer. The sludge samples considered in this study included kaolin, alum and biological sludge. The binding strength of bound water at -20°C was calculated as 24.5 kJ/kg, which is close to that of the normal physico-adsorption process. Most of the frozen moisture in a dilatometric measurement should be interstitial water or vicinal water. Results also indicated that moisture with a lower binding strength easily became free water. The release of interstitial water from sludge flocs would cause less bound water. When solid concentration is higher, the expelling strength induced by the compacting action of sludge flocs is increased over the binding strength, thereby forcing out the interstitial water. Lower bound water contents are attained. Accurate measurement of bubble volumes with the calculated stem on the top of the dilatometer could improve the precision of expansion coefficient and bound water content measurement.