The chemical doping of monolayer and bilayer graphene with aluminium, silicon, phosphorus and sulfur was investigated. Si-doped graphene has the lowest formation energy although it is semimetallic. P-doped graphene has a magnetic moment of 1 mu(B) and for 3 at.% of doping the band gap is 0.67 eV. Al-doped graphene is very unstable but it is an attractive material because it is metallic. To reduce the formation energies of the substitutional defects we investigated the formation of interlayer bonds in bilayer graphene. Phosphorus forms the strongest bonds between layers giving particular stability to this material. P-doped bilayer graphene has a gap of 0.43 eV but it is has no magnetic moment. (C) 2010 Elsevier B.V. All rights reserved.