Some soils of the state of São Paulo possess a certain amount of bases that cannot be considered as exchangeable. The behavior of exchangeable and non-exchangeable bases in some soil types was studied by means of cation extraction with nitric acid solutions of the following concentrations: 0.005 N, 0.01 N, 0.05 N, 0.075 N, 0. 5N, 1 N, and 2 N. The exchangeable cations in the samples were determined by leaching the soil with normal ammonium acetate at pH 7, using for aliquots of the same samples 100, 200, 300, and 400 ml of the solution. Various soil types were used in these studies, including the latosols, red-yellow podzolic, and "terra-roxa", all with low content of bases. The results obtained permit the following conclusions: a) all concentrations of nitric acid tried removed the same amount of bases as was removed by the ammonium acetate method; the content variation leached by the nitric acid was identical for any volume of ammonium acetate used; b) it was confirmed that acid solutions may be employed for the determination of bases in semi-tropical soils as those of the state of São Paulo; c) non-exchangeable bases were not affected by acid solutions as strong as 2 N; d) the exchangeable property is characteristic and remains constant over a large range of acid solution concentrations; e) since the uptake of bases by plants is considered an exchange phenomenon and it is hard to conceive that plant roots may create more energetic conditions than those resulting from 2 N nitric acid treatment, it is assumed that only exchangeable bases in these soils are available for plant absorption. The amount of individual exchangeable bases is thus critical for the study of soil fertility.