The aggregation behavior of ultrafine alumina in surfactant solution is examined by particle size analyzer. Electrokinetic and contact angle measurements are used to discuss the adsorption mechanism of sodium dodecyl sulfate (SDS) and dodecylamine (DDA) chloride at alumina. The PZC value of alumina is 9.1. SDS and DDA made alumina surface hydrophobic, respectively at pH < PZC and pH > PZC, and hydrophobic aggregation between alumina particles take place. The interfacial interaction energies between alumina particles in solution have been obtained based on polar interfacial interaction theory and contact angle measurements. The classical DLVO theory only considering the electrostatic and van der Waals interaction fails to explain the hydrophobic aggregation of ultrafine alumina, which can be well explained in the extended DLVO theory concerning polar interfacial interaction. (C) 2003 Published by Elsevier Ltd.