Industrial companies and public waste management agencies envision clay-rich materials as efficient barriers for large-scale confinement of nuclear waste and subsurface CO2. In clays, small pores hinder water flow and make diffusion the dominant solute-transport mechanism. Most clay mineral structures exhibit a negative charge that is balanced by an electrical double layer at the mineral water interface. This clay mineral property delays cation migration through adsorption processes, decreases the accessible porosity and diffusion fluxes for anions compared to those of water and cations, and gives rise to semipermeable membrane properties. Here we present experimental data that demonstrate for the first time that anions can be completely excluded from the smallest pores within a compacted illitic clay material, an observation that has important implications for the ability to accurately predict the containment capacity of clay-based barriers. In a series of multitracer diffusion experiments, celestite (SrSO4) precipitation reduced the porosity of compacted illite to the point where the water tracer diffusion flux decreased by half, while the chloride diffusion flux decreased to zero. This result demonstrates that anions can be completely excluded from the smallest pores within a compacted clay material.