Soil pH measured in 1 m NaF (pH_NaF) can be a useful tool for soil classification and to provide better advice on the chemical management of agricultural soils in the tropics. In this study, we verified the effects of clay mineralogy on pH_NaF values of non-allophanic soils of São Paulo State, Brazil. Fourteen subsurface soil samples were characterised for chemical properties, clay content, clay mineralogy, and for pH_NaF values, which were measured in the whole soil and in both natural and deferrified (dithionite-treated) clay fractions. Regression and correlation analyses showed that both ammonium-oxalate-extractable Al (Al_o) and gibbsite contents have positive relationships with both clay and soil pH_NaF values. On the other hand, kaolinite is inversely related to the pH_NaF measured in the clay and has nonsignificant effect on soil pH_NaF. X-ray diffraction patterns of dithionite-treated clays did not show disruption of kaolinite or gibbsite after the treatment with 1 m NaF, suggesting that the displacement of surface OH groups by F^– seems to be the main mechanism associated with the pH increase verified in the NaF solution after its contact with the deferrified clay fraction. The smaller influence of hematite on pH_NaF seems to be due to its correlation to Al_o. Goethite and ammonium-oxalate-extractable Fe (Fe_o) exert no effect on pH_NaF. Finally, the relationships observed in the present study strongly suggest that pH_NaF values <10.3 measured in non-allophanic kaolinitic soils with low levels of non-humified organic matter are essentially due to their smaller Al_o and gibbsite contents, which agrees with the direct correlation verified between pH_NaF and soil weathering degree.