Technical Abstract: The chemistry of saline soils differs significantly from that present in non saline, predominantly humid environments. Among the differences are the importance in saline soils of the process of evapotranspiration, soil mineralogy, with predominance of smectites over kaolinite, elevated pH and base cation saturation of exchange sites. Calculation of solution activities in saline soils is essential for all solution speciation and solubility calculations. Discussion is presented of the different approaches to activity corrections and the implications for calculation of saturation status and controlling phases. Major solid phases controlling solution composition are reviewed including calcite, gypsum and dolomite. Other phases such as mirabilite, sepiolite and highly soluble salts control solution composition under yet more saline conditions. A chemical evolution scheme is presented outlining the development of different chemical end member compositions upon concentration of the solution.