The formation of thermoreversible gels from aqueous aluminum polyphosphate solutions was previously assigned to hydration-dependent interionic associations, but these were not elucidated. In the present work, aluminum-phosphate interactions were examined by P-31- and Al-27-NMR spectroscopy of aqueous aluminum polyphosphate solutions and gels, at various P/Al ratios and temperatures. The fraction of aluminum and phosphate groups actually involved in the sol-gel transition is small, as evidenced by the comparison of spectra of solutions at various P/Al ratios and temperatures, in which only minor spectral changes are actually associated with gel formation. Chemical groups responsible for network formation are mainly Al(H2O)(4)(PO3)(2) groups, in which the phosphate ligands are polyphosphate chain-ends. Within the gels, a large fraction of total aluminum is found as hexaquoaluminum ions, as evidenced by Al-27-NMR spectra; on the other hand, P-31-NMR spectra show that most phosphate ions are associated. These data indicate that bonding and bridging involve both inner- and outer-coordination sphere interactions. The former depend on ligand exchange reactions with half-life on the order of magnitude of a few seconds, which explains the time dependence of thermoreversible sol-gel transformations, in this system.