The impact of gas slippage on experimentally-derived permeability effective stress laws has been investigated using gas shale from the Montney Formation, Canada, and previously published data for gas shales from the Eagle Ford and Marcellus Formations, USA. In these low permeability reservoirs, gas slippage results in apparent permeability effect stress law coefficients less than one, and drives coefficients towards lower values at higher confining pressures and lower pore pressures. The results show that in low permeability rocks, such as shale gas and shale oil reservoirs, gas slippage is significant at pore pressures in excess of 7 MPa, and if not considered results in significant errors in calculated effective stress law coefficients. Permeability effective stress laws reported in the literature for fine-grained lithologies under the assumption that gas slippage was negligible at and above pore pressures of 7 MPa are likely in error. Such studies need to be thoroughly re-examined, and future studies of fine-grained lithologies need to be conscious of significant apparent permeability variation due to gas slippage at pore pressures at and above 7 MPa. (C) 2016 Elsevier B.V. All rights reserved.