In soil environments, particularly in the soil-plant interface, organic and inorganic ligands may interact each other for sorption sites onto soil components. The concentration of sulfate in Andisols and Ultisols of Southern Chile has been diminished because the presence of phosphate and organic acids in the soil solution, which influence sulfate leaching (Mora et al., 2005). The aim of this research was to describe the sulfate adsorption mechanisms and the interaction with organic acid and phosphate on an allophanic synthetic compound by means of mechanistic models, thermodynamic parameters under controlled conditions of temperature, ionic strength and pH. Sulfate adsorption experiments were carried out on allophane synthetic compound coated with iron oxide (AlSiFe), similar to allophanes present naturally in volcanic soils. The data of sulfate adsorption were modeled by the Triple Layer Model (TLM) considering outer and inner sphere complexes. The TLM was able to describe the sulfate adsorption by inner and outer sphere complexes. The surface complexation constants were i) logK_out,1 = 12.39, logK_out,2 = 9.06 for outer sphere complexes and ii) log K_inn,1 = 14.25, logK_inn,2 = 10.80 for inner sphere complexes. The thermodynamic parameters determined using experiment with variable temperature and previous results by TLM showed values of: i) DG_abs between -94.0 and -44.6 kJ mol^-1 (spontaneous reaction), ii) DH_abs between 3.1 and 23.5 kJ mol^-1 (endothermic reaction) and iii) DS_abs between 154.4 and 394 J mol^-1 K^-1 (high entropic contribution). On the other hand, sulfate adsorption was strongly inhibited in the presence of equimolar amounts of oxalate, citrate or phosphate even at low surface coverage of all the ligands. The results seem to demonstrate that changes in the electric potential of the surfaces after the addition of anions, as well as competition for adsorption sites, affected the reduction in adsorption of sulfate and the other ligands. However, competition in adsorption with sulfate for common adsorption sites was greater for the organic ligands than for phosphate, because the surface coverage of phosphate was particularly low with respect to that of sulfate, citrate and oxalate. Therefore, the mechanism of the sulfate adsorption was interpreted through the formation of an activated complex on the protonated surface, and later an inner or outer complex is formed on the surface. sulfate was adsorbed mainly as an outer sphere complex and approximately 10 to 20 % as inner sphere complex, which depended on the temperature, pH and ionic strength conditions and the presence of ions in soil solution. Acknowledgement: FONDECYT 2000110. References: (1) Mora, M.L., Shene, C., Violante, A., Demanet, R. y Bolan, N.S. 2005. The effect of organic matter and soil chemical properties on sulfate sorption in Chilean volcanic soils. p. 444. (2) In Huang, Violante, Bollag and Vityakon (eds) Soil Abiotic and Biotic Interactions and the impact on the ecosystem and human welfare.