A waxy, a dent, and a high amylose starch were subjected to thermomechanical processing of varying degrees of severity. These treatments ranged from gentle dissolution in 90%DMSO to autoclaving, at temperatures up to 180 °C, to jet‐cooking. The treatments had major effects on viscosities of the starch dispersions and in particular on the intrinsic viscosities, which were determined to provide a relative measure of the molecular sizes of the processed starches. The intrinsic viscosities, from the least severe treatment to the most severe treatment, were 220 and 91; 181 and 104; 105–80 ml/g, respectively, for the waxy, dent, and high amylose starches. An unexpected characteristic of processed waxy maize was that the viscosity of 6% dispersions was extremely sensitive to intrinsic viscosity changes in the narrow range of 180–165 ml/g. In contrast, viscosities of waxy maize dispersions were relatively insensitive to changes in the lower intrinsic viscosity values (<160 ml/g) caused by more severe treatments. Dispersions of jet‐cooked waxy maize exhibited Newtonian flow; jet‐cooked dispersions of high amylose starch did not.