The primary electroviscous effect has been investigated in dilute suspensions of titanium oxide (anatase), the viscosities of which were measured by means of a capillary viscometer with automatic timing. The linear relation between viscosity and solids volume fraction was first determined at the isoelectric point of the particles when the particles are uncharged, and the electroviscous contribution to the intrinsic viscosity was then determined at other values of pH. Booth's theory (Proc. R. Soc. London Ser A 203, 533 (1950)) agrees well with the experimental results when the particle zeta potential is small and the double layer is thin (kappaa approximate to 7.3), but agreement is poor when the double layer is thick (kappaa approximate to 0.6).