The behaviour of exchangeable aluminium and hydrogen in the broad range of pH was studied at model conditions on clays taken from acidic soils of different origin. Sodium and aluminium forms of clays were used and ion exchange experiments were performed. On the basis of the results obtained and data previously reported, it was calculated that exchangeable aluminium below pH 4 is trivalent and in the pH range between 4 to 6 its average charge decreases. Measurable, exchangeable hydrogen quantities occur only at very low pH values.