Polynuclear Al 13 tridecamer species are the major hydrolyzed species of aluminum, but their occurrence in terrestrial environments has not been established. X-ray diffraction (XRD), 27 Al nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) analyses show that the presence of tartaric acid (concentration range of 10 (super -5) -10 (super -3) M), one of the commonly occurring low-molecular-weight organic acids, inhibits the formation of the Al 13 tridecamer species. In the absence of tartaric acid, the basic aluminum sulfate crystals were of tetrahedral morphology and conformed to isometric symmetry with a = 17.748 Aa and space group of P4 2 32. Increasing amounts of tartaric acid [tartaric acid/Al molar ratio (R) ranging from 0.01 to 0.05] modified the crystal morphology from the tetrahedral particles of isometric symmetry (R = 0) to rod-shaped particles of monoclinic symmetry (R = 0.01) to irregularly shaped X-ray noncrystalline microparticles (R = 0.05). Failure to detect the presence of Al 13 tridecamer, the dominant hydrolyzed species of aluminum, in terrestrial environments may be partially attributed to the presence of low-molecular-weight organic acids, which inhibit the formation of Al 13 tridecamer species.