The determination of Al is of permanent concern in soil fertility management. Soil chemists are constantly trying out methods that take into account the effective exchange capacity of the soil, the organic matter content, the texture, and the pH-dependent charge of various clays minerals. In Brazil, exchangeable Al is obtained by titration of KCl filtrate with NaOH. However, this is an indirect determination based on the general assumption that the stoichiometry of H⁺/Al³⁺ is around 3 (McLean, 1976). Thus, it was the objective of this study to investigate if interactions between Al and other soil components could alter the stoichiometry of H⁺/Al³⁺ exchange. The routine method for extracting Al in most Brazilian soil laboratories (Bernardi et al., 2002) is based on the exchange of Al from the surface of soil colloids by a non-buffered solution of KCl 1 mol L^-1, using a soil:solution ratio of 1:10. After one night, the Al is indirectly quantified by titration with NaOH 0.025 mol L^-1. For this study, the Al in the extract was also directly measured using an ICP OES. Forty-nine acid soil samples were selected representing different soil classes (Oxisol, Ultisol, Inceptisol, Entisol, Histosols) from different Brazilian States (RJ, AM, MS, SP, AL, PB, PE, PR, MG, DF). The results could be arranged in 3 groups. The first one was composed of 19 horizons from soils where the main dominant clay mineralogy is kaolinite and/or iron/aluminum oxy-hydroxydes. In that case, the mean H⁺/Al³⁺ ratio found was around 3.4. Besides the errors of titration, the small contribution of Mn²⁺ and Fe³⁺ hydrolysis may be considered as the cause of the positive variation from the expected 3.0 ratio. The second group was composed of horizons from soils that showed considerable amounts of secondary 2:1 minerals and random interstratified minerals. In that case, the H⁺/Al³⁺ ratio was around 35.4. Despite the fact that Al content, determined by ICP-OES, was high, appreciable amounts of hydroxy-interlayers could contribute for increasing H⁺ titration. Thus, the amount of H titrated should be higher than the expected. The last group was composed of organic horizons with the H⁺/Al³⁺ ratio around 0.5. This discrepancy may be explained by the complexation of Al by organic fractions. Data on Al extracted with CuCl₂ corroborates this hypothesis. Thus, in that case, the forced hydrolysis of Al³⁺ by alkaline titration should be lower than the expected. The results obtained in the study showed that the interpretation of soil acidity that is due to Al may vary, affecting all recommendation of liming based on exchangeable aluminum.