Anaerobic conditions in landfills will be switched to aerobic ones, leading to a decrease in pH, due to the oxygen intrusion in the long-term perspective. Acid buffering capacity (ABC) of the clay liner-leachate system controlling this acidification process was estimated by acidimetric titration tests on Osaka marine clay dispersed in various solutions including distilled water and the leachate from two practical landfills. Test results indicate that the newly generated leachate from a MSW landfill have a higher ABC than the aged leachate. While the leachate from a MSW incineration ash landfill is characterized by a high ABC especially when pH>8.0.