Soil and stream water aluminium and hydrogen ion data are presented for three Scottish catchments that are both acidic and acid sensitive. Stream waters have pH values ranging from 3.9 to 7.1 and aluminium concentrations in the range 0.07 to 129 μM 1−1. Within the soils H+concentrations decline with depth, being highest in the organic horizons and lowest in the inorganic horizons. Mean H+concentrations vary markedly from site to site (H+ranges from 12 to 111 and from 9 to 72 μM 1−1in the upper and lower soils respectively). Correspondingly dissolved aluminium varies with depth and from site to site (range from 2 to 21 and from 8 to 17 μM 1−1in the upper and lower soils respectively). Thermodynamic analysis of these data sets show that simple Al(OH)3and Al(OH)SO4solubility relationships are inoperative in the stream and soil zones. Dis-equilibrium “weathering” processes are inferred, even in the case where Al(OH)3(gibbsite) is found within the catchment.