The influence of the rotation rate of a rotary viscometer on the photoelectrorheological (PER) properties of two types of TiO2 powders dispersed in silicone oil was investigated. Type P-25 TiO2 (water content, 1.2 wt%) particle suspensions exhibited a positive photoeffect on the viscosity, i.e. the combination of UV illumination and a de electric field produced a larger increase in the fluid viscosity than that produced by the electric field alone. The positive effect was enhanced with increasing rotation rate. Type ST-01 TiO2 (water content, 10 wt%) particle suspensions exhibited behavior in which there was a negative photoeffect (UV illumination produced a decrease in the held-enhanced viscosity) when the rotation speed was low (less than or equal to 50 rpm), and a positive photoeffect when the rotation speed was high (greater than or equal to 100 rpm). The difference in the behavior of these two types of TiO2 powders is proposed to be due to the higher water content of the type ST-01 powder. The presence of water facilitates the trapping of excess charge, giving rise to photoelectrophoresis, which inhibits the formation of bridge-like particle structures, and as a result, the additional fluid viscosity produced by the electric field again decreases as a result of illumination. For both fluids, the photoeffect at higher rotation rates was more positive (i.e. less negative in the case of ST-01) than that at lower rotation rates. The increased photoeffect with increasing rotation rate is most likely due to the decrease in the frequency of particle contacts with the electrodes, because such contacts are necessary in order to inject excess charge into the particles, giving rise to photoelectrophoresis. Evidence in support of the decreasing frequency of particle contacts with increasing rotation rate was the decreased electrophoretic current flowing between the electrodes. In order to explain the dependence of the photoeffect on the rotation rate and water content, we have proposed a model in which photoelectrophoresis reduces the net attractive force between particles.