|The induction of crassulacean acid metabolism(CAM) characterized by day/night acid fluctuation was investigated in leaves from 10 days exposure to elevated CO_(2) concentration(1,000 μL L^-1 CO_2). For Pereskia aculeata, have C_(3)-like gas exchange pattern in well watered condition and shift into CAM-like in water stress, showed a more typical CAM-like diurnal acid fluctuation. Whereas the massive diurnal fluctuation of acidity in typical CAM of Kalanchoe rosea was declined. The effect of short-term exposure to various concentrations of CO_(2) on titratable acidity in P. aculeata and K. rosea was also investigated. To investigate the response of various CO_(2) concentrations, four different CO_(2) levels(350, 700, 1,000 and 1,500 μL L^-1) were imposed for 24hr and measured the titratable acidity at 06:00, when the acidity was maximum, and 14:00, when the acidity was minmum. The accumulation of acid in P. aculeata was enhanced markedly by higher concentration of CO_(2), while the level of acidity in K. rosea did not highly respond to CO_(2). A notable difference between P. aculeata and K. rosea was the response of de-acidification to a higher concentration of CO_(2). Increasing with CO_(2), the degree of de-acidification of P. aculeata was increased while that of K. rosea was depressed.