Graphene forms an important two-dimensional (2D) material class that displays both a high electronic conductivity and optical transparency when doped. Yet, the microscopic origin of the doping mechanism in single sheet or bulk intercalated systems remains unclear. Using large-scale ab initio simulations, we show the graphene surface acts as a catalytic reducing/oxidizing agent, driving the chemical disproportionation of adsorbed dopant layers into charge-transfer complexes which inject majority carriers into the 2D carbon lattice. As pertinent examples, we focus on the molecular SbCl5 and HNO3 intercalates, and the solid compound AlCl3. Identifying the microscopic mechanism for the catalytic action of graphene is important, given the availability of large area graphene sheets, to spur research into new redox reactions for use in science and technology. © 2011 American Chemical Society.