Water in sludge appears to be of four kinds; free water which freezes at normal freezing temperatures; interstitial water, which freezes at lower temperature due to high dissolved solids concentrations; vicinal water which is associated with solid surfaces and freezes only at very low temperatures; and water of hydration, which does not enter the ice crystal and which can be removed from the sludge only by thermal means. Using the classical technique of freezing in dilatometers, unfrozen water is measured in sludge at different temperatures and at different solids concentrations. Not all water appears to be frozen even down to −30°C. At low solids concentrations, where the sludge does not form large flocs, the only unfrozen water is most likely vicinal water and the water of hydration. At intermediate solids concentrations, the flocs capture the interstitial water which does not freeze, apparently due to its high dissolved solids concentration. At high solids concentrations, the interstitial water is squeezed out of the flocs and only the vicinal water and the water of hydration remain unfrozen. It is not possible to remove vicinal water or the water of hydration by mechanical means, but it is theoretically possible to remove the interstitial and free water. A knowledge of these fractions of water can provide practical limits of mechanical dewatering.