Soluble aluminum silicate (Al-Si) complexes are critical species for the development and application of geochemical models. A fluorescence probe technique was used to show that Al-Si complexes account for up to 95% of the total inorganic mononuclear Al in natural waters. The presence of these soluble Al-Si species affects the solubility of Al-Si minerals. Soluble Al-Si species may be important intermediates in the weathering reactions of primary Al-Si minerals and should be considered in dissolution models. Soluble Al-Si species may be key components controlling the formation of metastable and new stable minerals during interactions between water and rocks or soils.