The streaming potential technique has been used to obtain the zeta potentials of ThO2 in HCl, HI, HNO3, and H2SO4 of various concentrations. The data are correlated with observed settling rates and tendency toward ThO2 colloid formation. Streaming current measurements were made on type 304 stainless steel samples. Evidence is presented to demonstrate the significance of the zeta potentials of the ThO2 and the stainless steel in the formation of thin films and thick cakes of ThO2 on the stainless steel.