It has been assumed that the sensitivity of several phosphate-binding enzymes to fluoroaluminate complexes was due to a similarity of the AlF4- complex with the tetrahedral phosphate anion. Depending on the stoichiometry, the ligand nature, and the medium conditions, aluminum may form either octahedral or tetrahedral complexes with molecular ligands. An exhaustive NMR (Al-27,and F-19) study of aqueous solutions of fluoride and aluminum ions from pH 2 to pH 11 and for [F]/[Al] ratios up to 14 permits the characterization of the fluoroaluminate complexes observed. This study provides evidence of complexes at pH less than or equal to 6 containing respectively one and two fluorine atoms per aluminum, the other aluminum ligands being H2O. Contrary to previous results, assignments of the resonances to complexes corresponding to three or more fluorines per aluminum was difficult over the range of temperatures studied. This is due to rapid exchange processes between H2O and F- ligands. At pH higher than 2, it has been possible to assume the existence of complexes binding one or two OH- per aluminum with three or more fluorines undergoing a fast exchange. A variable-temperature F-19 NMR study pinpoints the pH-dependent exchange conditions between OH- and H2O ligands for the complexes possessing three or more fluorine atoms per aluminum. The present study also permits the conclusion that all the fluoroaluminate complexes observed in aqueous solution are hexacoordinated with an octahedral geometry. The equilibrium constants K-AlF1 to K-AlF3 have been experimentally determined.