The mechanisms of SO4 adsorption on clays have been investigated by many researchers. However, few researches have focused on the fraction of SO4 that is adsorbed in the diffuse layer to the total adsorbed SO4. We investigated SO4 adsorption in detail on an allophanic Andisol (volcanic ash soil.), especially the fraction of SO4 adsorbed in the diffuse layer to the total adsorbed SO4, conducting experiments under conditions of low pH (pH 3.3 and 4.3) and low ion concentrations (1.0 and 0.1 molc m^[-3]) to avoid a strong negative surface charge of the soil particles. SO4 and NO3 adsorption under their competitive conditions were measured by a batch method using mixtures of HNO3 and H2SO4. Exchangeable SO4 and NO3 were extracted with 1000 molc m^[-3] KCI. Strongly adsorbed SO4 was extracted with 10 molc m^[-3] NaOH after the extraction with 1000 molc m^[-3] KCI. The exchangeable SO4 made up to 72 to 77% of the total adsorbed SO4. These results suggested that both inner-sphere and outer-sphere complexes co-exist in the allophanic Andisol at low pH. SO4 was strongly selective over NO3 under these conditions. We compared adsorbed amounts calculated by the Gouy-Chapman model with the measured values at solution conditions of pH 3.3 and 1.0 molc m^[-3]. The model overestimated NO3 adsorption and underestimated SO4 adsorption. The difference is due to the fact that SO4 adsorption in the Stern layer is neglected. Next, we calculated SO4 adsorbed in the diffuse layer using the Stern-Gouy-Chapman model under the assumption that all the measured NO3 adsorbed was in the diffuse layer. Our results indicated that the SO4 in the diffuse layer made up only less than 6% of the total adsorbed SO4. Most of the adsorbed SO4 is likely to be found in direct contact with the soil surface.