SummaryHomoionic clay suspensions were equilibrated at 25°C with aqueous NH, (0.01 to 0.30 M) by agitating for several days. Using clays extracted from three different soils, sorption of NH3, measured by titration, followed the order Na>Ca>NH4‐clay. The difference in NH3sorption between Na‐clay and NH4‐clay was matched by an increase in Na+ion concentration in the aqueous phase of suspensions of Na‐clay treated with NH3, but there was no such effect with Ca‐clay.Sorption of NH3on NH4‐clay in aqueous suspension is considered due to proton donation at sites of pH‐dependent charge, and the additional sorption on Na‐clay to displacement of Na+by NH4+ions. The additional sorption on Ca‐clay seems largely due to co‐ordination of Ca2+ions with molecular NH. Exchangeable cations affect NH3sorption differently when an aqueous phase is present than when dried homoionic materials are