Based on the analytic data of soil samples collected from 257 soil layers of 66 soil profiles under 7 soil types in Tibet, characterist ics of the acidity of the acid soils were explored. Acid soils, accounting for one fifth of the total land area of Tibet, are mainly distributed in the southeast of Tibet, under the vegetation of mountain humid forests and alpine shrubby meadows. Soil pH is determined mainly by exchangeable acid in potential acid, especially, exchangeable Al³⁺, which is in dominancy, while non exchangeable acid has less impact on that. The proportion of exchangeable Al³⁺ increases with the in crease in exchangeable acid, but decreases with the increase in OM. However, the relative proportion of exchangeable H⁺ shows a reverse trend Non exchangeable acid comes mainly from OM (through disassociation of H and Al from humic acidic functional groups). Soil pH is in near linear negative correlation with the saturation of exchangeable acid and in non linear one with the saturation of nonexchangeable acid, but in nonlinear positive correlation with base saturation, which is mainly restrained by the dominant exchangeable base Ca²⁺.