The ability of pig gastric mucus to retard diffusion of hydrogen and other small ions was examined. Mucus layered between gauze supports separating equimolar solutions of hydrochloric acid and sodium chloride markedly delayed the diffusion of hydrogen ions when compared with the gauze supports alone acting as a barrier. This observation was confirmed using an isotope technique. The diffusion coefficient for hydrogen ions through a known thickness of mucus was four times smaller than the diffusion coefficient through an equivalent thickness of unstirred solution. Mucus probably delays the trasfer of hydrogen ions across it by maintaining an unstirred layer of water and in part by immobilizing some water in its interstices making it unavailable for diffusion, a characteristic which is dependent on its viscosity. These obervations may be relevant to the ability of gastric mucus to protect the underlying mucosa from acid.