One area where water-based muds need improved performance is in shale inhibition. However, before existing mud systems can be improved, the mechanisms by which water invades shales and how present-day inhibitive additives operate must be fully understood. An experimental technique has been developed that uses radioactive tracers to monitor the progress of water and selected dissolved ions through a shale core plug. By varying experimental parameters, such as water composition and applied pressure drop, the dominant mechanisms by which water is transported through shales have been identified. Under conditions of zero applied pressure, diffusion processes control water and ion movement through shales. Concentration gradients are the driving force for mass transfer of ionic species through shales. We observed no evidence to indicate that osmosis caused mass transfer of water. Applied pressure caused an increase in water and ion transport rates. Above a threshold pressure, water and dissolved ions travel at the same rate irrespective of the ion concentration.