It was the objective of this study to examine ultrafiltration of gelatinized starch to compare with other viscous streams and to develop a supporting data base. The objective was broadened to explain and model observations contrasted with other viscous materials. In particular, the results of ultrafiltration of starch are not consistent with diffusion-based theories of ultrafiltration; however, the resistance to flow is well modeled as a flowing filter cake. The results presented here may be useful for predicting the performance of a starch reactor wherein the goal is to convert the starch into sugars or alcohol via the action of special enzymes or hydrolysis.