The electrorheological properties of a TiO2 (anatase) particle suspension in silicone oil were found to undergo changes during illumination, including an increased electrorheological effect, i.e., increased viscosity upon illumination. The increased viscosity is due to the polarization of photoinduced charge carriers under the influence of the imposed electric field, A small photocurrent was observed, which can be attributed to photoelectrophoretic conduction and leakage of photogenerated charge through field-induced columns. Microscopic observation carried out under stationary conditions showed an increase in the number of field-induced columns upon illumination, consistent with increased viscosity.