Hydrogeochemical cycles couple atmosphere, land and water; here the effect of these cycles on the distribution of chemical species and the regulation of the composition of natural waters is discussed. Above all, the regulation of the delicate proton balance is emphasized, which in turn regulates the solubility of minerals, the availability of nutrients and the speciation of metal ions. In assessing the anthropogenic effect on the proton balance, it is important to distinguish between the H⁺, the H⁺ ion reservoir as given by the base neutralizing capacity (acidity) and the acid neutralizing capacity (alkalinity). Rigorous conceptual definitions of these neutralizing capacities are given. The effects of biological processes (production and mineralization of biomass, biological mediation of redox processes such as nitrification, denitrification, SO₄^2- reduction), and of weathering reactions on alkalinity are discussed. Chemical weathering (which consumes protons and produces alkalinity) is one of the major processes controlling the hydrogeochemical cycles of elements. The chemical erosion rate in the upper Rhine catchment area is exemplified. It is shown how concepts in the chemistry of the mineral-water interface can be used to predict surface reactivity and to account for the variables (pH, ligands) which influence the dissolution of minerals.