Photoeffects on the electrorheological (ER) properties of various types of commercial TiO2 powders dispersed in silicone oil were investigated. Two types of photoelectrorheological (PER) effects (i.e., positive and negative) were observed in these fluids, the effect being dependent on the adsorbed water content of the powder. The positive PER effect of low-water-content (<2 wt %) particle suspensions during illumination, associated with low photocurrents, is due to the polarization of photogenerated electrons and holes under the influence of the applied electric field, which facilitates the formation of particle cluster bridges between the electrodes under the relatively low flow rate conditions used, in this work. The negative PER effect in the case of high-water-content (>3 wt %) particle suspensions is correlated with the high degree of photoelectrophoretic (PEP) oscillatory motion of particle clusters;between the electrodes that was directly observed and measured using microvideo techniques under stationary conditions (i.e., no net flow). This circulation of clusters is responsible for the high photocurrents that are observed even under flowing conditions, that is, when there are few bridges. The enhancement of the PEP effect due to the presence of water is explained on the basis of (1) enhanced charge transfer from the electrode to the particles and (2) greater numbers of trapped photogenerated carriers. These carriers can be trapped via reactions involving water, and the resulting species can be either reduced at potentials less negative than the conduction band edge or oxidized at potentials less positive than the valence band edge. The large decrease in the residence time of particles at the anode due to illumination is thought to be due to a large increase in the concentration of photogenerated minority carriers (holes) in the TiO2, which is an n-type semiconductor. The enhanced PEP effect which results from increased water content is believed to be a key factor in the diminution of the ER effect in the high-water-content particle suspensions.