Aluminum and hydrogen ions extracted from soils with 1 N KC1 solution are considered the most active forms of acidity. In fact, the displacement of those ions by continuous percolation with 250 ml of 1 N KC1 solution increased the pH of seven soils samples studied, determined in soil: water proportion 1 : 2,5. However, the pH of five soils samples remained 6.25, 6.05, 5.80, 5.45 and 5.05 after the extraction with 1 N KC1 solution, pointing out the presence of other active forms of acidity. Those samples wich were treated with KC1 solution and then with 1 N calcium acetate solution(pH=7,0) presented other components of soil acidity, determined by titration with standardized 0,02 N NaOH solution.