Department of Chemistry, Gandhigram Rural Institute-Deemed University, Gandhigram-624 302, India E-mail : drg_karthikeyan@rediffmail.com Fax : 91-451-2454466 Manuscript received 24 April 2003. revised 14 October 2003, accepted 14 january 2004 The defluoridation efficiency of the commercially available grades of activated alumina were determined. The various factors which influence the defluoridation efficiency of the material were studied. There was a marked dependence of pH on the defluoridating capacity. The adsorption of fluoride ions on activated alumina was studied in the temperature range of 303-333 K by the batch equilibration method. The experimental adsorption data fitted well to the Langmuir isotherm indicating that the forces of adsorption are governed mainly by chemisorption. The equilibrium parameters and activation parameters were calculated and reported. The results indicate that the adsorption process is endothermic and occurs spontaneously which substantiate the proposed mechanism for the adsorption of fluoride at the alumina-solution interface.