Complex fluids whose viscosity can be tuned by light have attracted significant attention recently. Such fluids could be useful in creating valves or sensors within microfluidic or MEMS devices. Our group has been interested in developing low-cost photoresponsive systems that do not require costly photosurfactants or polymers. Recently, we have reported a photo-thinning fluid based on a mixture of a common cationic surfactant and a photosensitive organic molecule (JACS 129, 1553 (2007)). The concept underlying the light-induced effect in the above fluid was based on variations in the length of worm-like micelles. Here we will present a low-cost photogelling system based on a different concept, viz., light-activated assembly of nanoparticles into a gel. We initially combine nanoparticles with a surfactant and a photoacid generator (PAG), creating a stable, low-viscosity dispersion. Upon UV irradiation, the PAG gets ionized, lowering the pH by about 3 units. In the process, we find that the particles assemble into a 3-D network, thus giving rise to gel-like behavior. We will report studies from a variety of techniques including rheology, SANS, and UV-Vis spectroscopy on our photogelling dispersions.