Graphite oxide (GO) is a promising precursor for the bulk production of graphene-based materials due to its relatively low cost of synthesis. The superior solvent processability of GO makes it particularly attractive for making composites by premixing with other materials. Typically, the reduction of GO has relied on either chemical agents or high temperature treatment. Here we report a room temperature, chemical-free flash reduction process where a photographic camera flash instantaneously triggers the deoxygenation reaction of GO by photothermal heating. Flash irradiation also rapidly creates a fused polymer composite from a random mixture of GO and polymer particles. Using a photomask, conducting patterns such as interdigitated electrode arrays can be readily made on flexible substrates.