Polyphosphates are widely used as inorganic dispersants, but the mechanism of the dispersing is not well understood. In this study, the colloidal properties of fine alumina suspensions in water were studied at different pH conditions in the presence of linear chain polymetaphosphates. The chemical speciation and molecular conformation were probed using liquid and solid state P-31 and Al-27 nuclear magnetic resonance spectroscopy. Three regimes have been found : (i) Neutral pH-long-chain metaphosphates are partially anchored to the alumina surface. The negatively charged metaphosphate molecules extend from the particle surface to the solution to provide an effective electrosteric repulsion force that acts to disperse the colloidal particles. (ii) Low pH-the alumina particle surfaces hydrolyze and the dissolved species react with the surfactant to form amorphous aluminum phosphate gels. The reaction product causes bridging flocculation of the suspension. (iii) High pH-partial dissolution of alumina produces tetrahedrally bonded hydroxyaluminate anions, which induces the long-chain polymetaphosphates to dissociate into phosphate anions. Under these conditions, the metaphosphate no longer exhibits dispersion properties. A fundamental understanding of the particle-surfactant interactions in terms of adsorption, chemical reaction, and molecular conformation will provide insights for the rational design of more effective inorganic dispersants.