The presence of water in mudrocks has a largely negative impact on production of gas stored in these rocks, due to the fact that water causes swelling of the rock. Removing the water from the mudrock could potentially shrink the rock and increase the overall permeability of the rock. Investigation of the swelling/shrinkage behaviour of the rock during exposure to water vapour is of key importance in designing and optimizing unconventional production strategies. We have used outcrop samples of the Whitby Mudstone and the Posidonia shale, potential unconventional sources for gas in North-western Europe, to measure the swelling and shrinkage behaviour. Swelling and shrinkage of the rocks when exposed to water vapour was measured directly using 1 mm sample cubes in two different setups. The mm cubes were exposed to different levels of relative humidity either in an Environmental Scanning Electron Microscope (ESEM) or in a 3D dilatometer. Swelling of Whitby Mudstone and Posidonia shale is heterogeneous with 2–3 times more measured swelling strain perpendicular to the bedding. Volumetric swelling strains showed values between 0.6 and 2.2% for the Whitby mudstone and the Posidonia shale, respectively. The results suggest that it might be possible to increase permeability in the reservoir by decreasing the in-situ water activity due to shrinkage of the matrix.