Soil solid phase and soil solution characteristics were evaluated on surface and subsoil horizons of 60 acid, highly weathered soils in the unamended state, and on a subset of 39 surface horizons following the addition of CaSO4.2H2O. The activity of Al3+ Was found to be strongly correlated with the soil solution pH, with Al3+ activity increasing as pH decreased. For surface soils in the unamended state, and for the majority of subsoils, the Al3+ activity calculated was comparable to the theoretical activity supported by the dissolution of gibbsite (Al2O3.3H2O). The Al3+ activity in CaSO4.2H2O-amended samples tended to fall below that maintained by gibbsite dissolution and was more closely correlated with the Al3+ activity supported by the dissolution of jurbanite (AlSO4 (OH).5H2O).