In this study, graphene-based titanium dioxide and zinc oxide composites (TiO2-G, ZnO-G) were synthesized using a hydrothermal process. Materials were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-vis spectroscopy, scanning electron microscopy, and transmission electron microscopy. Photocatalytic activity of the composite materials under simulated solar light was studied using phenol as a model compound. A ca. 30% improvement on the degradation performance by the TiO2-G composite (and ZnO-G) was observed when reaction rate constants were compared with TiO2 (and ZnO) only. This demonstrates the positive effect of graphene on suppressing charge recombination and extending the light absorption range. Further improvement on the photocatalytic degradation rate of phenol was obtained by coupling the two composites, ZnO-G and TiO2-G. This is attributed to more efficient charge separation and longer lifetime of the charge carriers, which eventually enhances the photocatalytic activity. The optimum stoichiometric amount of each component was obtained experimentally. Systematic parametric studies were also performed to study the effect of catalyst loading, initial phenol concentration, solution pH, and solar light intensity. Complete solar degradation of 40 ppm phenol was achieved within 60 min while using the coupled ZnO-G/TiO2-G photocatalysts at the optimum conditions.