In this study, a transition from activated conduction mechanism to variable range carrier hopping conduction mechanism is observed on adsorption of H-2, Ar and O-2 molecules onto graphene layers in the temperature range from 173 K to 373 K. A fast switching (<100 s), recovery (<300 s), completely reversible behavior and lack of dependence on nature of the molecules show that physical adsorption of gas molecules results in appearance of defect states in the bandgap of graphene. These results provide further understanding of electrical conduction mechanism in graphene and are important for its applications in gas sensing and hydrogen storage devices. (C) 2016 The Electrochemical Society. All rights reserved.