Soil pH and base saturation are important parameters for evaluating the effects of acidity on plant growth. However, the relationship between soil pH and base saturation (V%) has been poorly studied in Brazil. The aim of this study was to evaluate the influence on the cation exchange capacity (CEC) on this relationship for soils of Mato Grosso do Sul, Brazil, using a database consisting of 30,244 analyses of soils collected from 0–0.2- and 0.2–0.4-m depths. The CEC at pH 7 of analyzed soils varied from 3.4 to 40.5 cmol_c·dm^-3 and were grouped into four classes (< 6, 6–10, 10–20, and > 20 cmol_c·dm^-3) for each soil layer. The V% versus pH relationship was CEC dependent for both soil layers. For 0–0.2 m, V% was directly related to CEC (< 6, 6–10, 10–20, and > 20 cmol_c·dm^-3) and reached 50, 55, 60, and 74% at pH 5 and 66, 73, 81, and 88% at pH 6, respectively. Predicting V% in relation to pH was also affected by CEC, with a decrease in the correlation of p = 0.82 for CEC < 6 cmol_c·dm^-3 to p = 0.64 for CEC > 20 cmol_c·dm^-3. The decrease in this correlation was related to low clay content (17%) of soils with CEC < 6 cmol_c·dm^-3, which provided low buffering capacity. For soils with CEC > 20 cmol_c·dm^-3 and higher clay content (45%), greater buffering capacity was observed associated with kaolinite. Findings suggest that CEC should be considered to understand the relationship between V% and pH CaCl₂.