While a variety of topologically nontrivial insulator phases have been predicted to arise from electron-electron and spin-orbit interactions in bilayer graphene, the trigonal warping of conduction and valence bands leads to a (semi) metallic band structure. An electrostatic potential difference between the two layers due to an external electric field is known to open a bandgap, leading to a topologically nontrivial insulator state. A bandgap may also arise from gas molecules adsorbed on bilayer graphene, implying a topologically nontrivial insulator phase. Here, our density functional theory calculations show that bilayer graphene adsorbing gas molecules is a quantum valley Hall insulator. Thus, adsorption of weak donor (or acceptor) molecules with a large electric dipole moment may be instrumental to realize a topologically nontrivial insulator phase in bilayer graphene even without external electric field. (C) 2015 Elsevier B.V. All rights reserved.