Synthetic anorthite has been used as starting materials and altered in aluminum sulfate solution under hydrothermal conditions. Various chemical behaviors are shown under the influence of temperatures and running times. At 200°C, leaching value of SiO₂ is smaller than at 100°C, being greatest at 300°C. Leaching value of A1₂0₃ at 100°C is larger than at 200°C. A1₂O₃ decreases in the early-middle stages (50-150 hours) and rapidly increases in the later stage (260 hours) of 300°C runs. In the early stage of 300°C runs, CaO decreases rapidly and then decreases gradually until 260 hours. Desilification progresses gradually at 100°C and 200°C runs, and in the early stage of 300°C runs, shows an upward tendency. Desilification declines rapidly in the middle stage, and then tends to show upturned curve once again in the later stage of 300°C runs. These chemical behaviors can be harmonized when the combinations of following fundamental alteration processes are involved: (1) decomposition of anorthite, (2) variation of mineral paragenesis (gypsum, anhydrite, kaolin and boehmite, etc.) in alteration processes, and (3) relative quantity of alteration minerals.