Electro-osmosis is an efficient means to move fluid in microfluidic channels. The flow is driven by the interaction of the electrical double layer at the channel wall with an electric field along the channel. The flow can be controlled by modifying the electrical parameters, either the charge of the channel wall or the electric field. If the surface chagre or the surface rsistance of the channel wall is sensitive to light, the flow can be modulated by light. We have demonstrated this effect by using photoconductive surfaces. The resistance change due to the illumination changes the electric field above the photoconductive layer and consequently changing the rate of fluid flow. By using channels where upon a photoresistive CdS surface a linear PDMS channel was placed, flow rate changes of an order of magnitude were achieved. This gives serious possibilities for optical control of flow. We further developed the method by building channel structures of more complicated patterns, e.g. Y-junctions. By appropriate illumination of the arms the flow direction could be selected between the arms optically. This unit is the basis of more complex flow patterns, it demonstrates the feasibility of optical control of such devices.