Solar light-induced photodecomposition of organophosphorus warfare agent simulant dimethyl methylphosphonate (DMMP) on the surfaces of TiO₂/graphene oxide (GO) nanocomposites was studied by in situ DRIFT spectroscopy. Nanocomposites containing 1 and 2 wt% GO, respectively, were prepared by two different aqueous methods. All nanocomposites were shown to effectively adsorb and partly dissociate DMMP as shown by the formation of surface coordinated methoxy groups. Solar light illumination induced rapid decomposition of the adsorbed species to yield various ionic and surface coordinated formate and carbonate species as the main intermediate products. Both the kinetics of adsorbed species decomposition and the formation of various intermediates were strongly affected by the presence of GO, as compared to pure TiO₂ nanoparticles. The two synthesis routes yielded nanocomposites with different degrees of reduction of GO that correlated with their reactivity towards DMMP adsorption and photodegradation. Upon illumination, rapid depletion of water was observed on the TiO₂/GO nanocomposites and was attributed to a water splitting reaction, which competed with the DMMP photo-oxidation reaction.