The effects of relative humidity on sorption and the diffusion of ethyl bromide in kaolinite and montmorillonite pellets were investigated by the one-sided single-pellet moment technique. Montmorillonite demonstrated a higher water affinity than kaolinite, causing a sharper decrease of the ethyl bromide adsorption equilibrium constant with an increase of relative humidity. For montmorillonite, most of the active sites are covered by water even at relative humidities of 8%, and adsorption of ethyl bromide on the gas–water interface controls the sorption process. For kaolinite, however, adsorption on gas-to-mineral, gas-to-water, and water-to-mineral surfaces contributes to the sorption process. Effective macropore diffusivity of ethyl bromide in montmorillonite showed a decreasing trend with increase in relative humidity. This is due to high water uptake of montmorillonite and the corresponding change in pore structure. No such change was observed for kaolinite.