The adsorption of an anion such as oxalate and phosphate at low concentration by soils is important because their contents in the soils are usually low. The adsorption of oxalate and phosphate on allophane at initial concentrations up to 200 μM showed that the amount of adsorption of phosphate was higher than that of oxalate, while the adsorption energy of oxalate estimated by the Freundlich parameter n was higher than that of phosphate. The high adsorption energy for oxalate was also suggested by molecular orbital calculations, where changes in the total energy of the adsorption reactions of oxalate were compared with that of phosphate. The amount of adsorption of oxalate or phosphate increased with raising the concentration of the background electrolyte (i.e. NaCl). This was ascribed to development of anion exchange site on allophane under the high concentration of NaCl. The amount of adsorption of oxalate coexisting with 200 μM of phosphate decreased as compared to that of oxalate alone. However, the adsorption strength of oxalate estimated by the Langmuir K and Freundlich n parameters was enhanced by the coexisting phosphate. For the adsorption of phosphate in coexisting oxalate, oxalate also had the effect on the adsorption strength of phosphate. Especially for the adsorption of oxalate, it was clearly observed that the amounts of Si and Al released from allophane with the adsorption abruptly increased at the equilibrium oxalate concentration of about 100 μM. This means that, at the equilibrium concentration, the adsorption schema of oxalate changed from the ligand exchange reaction between hydroxyl group and oxalate to the replacement reaction of silicate with oxalate. The critical equilibrium concentration shifted to about 70 μM with the addition of 200 μM phosphate. This indicates that the coexisting phosphate also consumed hydroxyls (Al-OH) leading oxalate to begin replacement with silicate at lower equilibrium concentration. It is concluded that, at low concentration, coexisting phosphate affect not only amount and strength but also mechanism of the adsorption of oxalate, and vice versa.