Several series of modified potato starches covering a wide range of fluidities were prepared by treatment with dilute hydrochloric acid at 50 °C. As in the case of previous results obtained with corn starches, it is shown that the viscosity is dependent on the rate of shear and that the data are well expressed by the exponential relation F = KPn, where F is the flow, P is the pressure, K and n are constants. The value of n decreases rapidly with increasing degree of modification. It is shown that n depends on the volume occupied by the swollen granules in the paste, and that n is much greater in potato starch pastes than in corn starch pastes owing to the greater swelling of the granules. This swelling decreases with increased degree of modification. The stability of the granules in alkaline pastes is described for both potato and corn starches, and the fluidities of alkaline potato pastes are shown to be much greater than those of corn pastes of comparable extent of modification. The gelatinization temperature is shown to rise with increasing modification. At the same time, the power of hydration of the granules decreases and they appear to become denser and more compact. The resistance to mechanical injury becomes greater with increasing modification, and the injured granules swell to a lesser extent. The decreased hydration (hydration plus immobilization) leads to different forms of breakage when the granules are crushed. Prolonged heating of starch pastes causes a rapid decrease in the value of n, owing chiefly to a disruption of the granules with consequent change in the phase-volume ratio.