Compressibility of colloidal cakes, which plays an important role in many engineering processes including deep-bed and membrane filtrations, is dependent on the properties of the colloidal particles and is affected by water chemistry. In this article, the compressibility of silica colloids was investigated by performing fouling experiments on a bench-scale ultrafiltration system. It was demonstrated that the cake compressibility of the deposited colloids on the membrane surface could be quantitatively related to the fouling potential of the colloidal suspension, which avoided the difficulty of measuring the specific cake resistance. Ionic strength was found to strongly influence the compressibility of the colloidal cakes. A linear relationship was found relating the cake compressibility to the natural logarithm of the feedwater ionic strength.