Gas is stored in shale mainly in free and adsorbed phases. Since a significant amount of gas in the shale is adsorbed to the organic matter and/or clay minerals, it is possible that gas adsorption will induce shale swelling, which may then has an impact on the gas flow behavior in the shale thus its gas production. In this work, strain behavior was studied in two gases, helium and methane, at different pore pressures under constant hydrostatic pressure at 20 MPa on two shale samples. The results show that porosity and volumetric strain are functions of gas pressure and the strain is larger in methane than helium demonstrating gas adsorption induced swelling for the shale samples. The calculated methane adsorption induced swelling strain is at a magnitude of 0.1% volumetrically with pressure at 10 MPa for the shale samples studied. The adsorption induced shale swelling strain shows a Langmuir-like relationship with pressure and is proportional to the amount of methane adsorbed. The results also show slight anisotropic strain behavior between the two directions of parallel and perpendicular to the bedding and strain hysteresis with methane in and out of the shale. The gas adsorption induced swelling may influence gas flow in gas shale, thus more research in this topic is warranted. (C) 2015 Elsevier B.V. All rights reserved,