The Lysina catchment in the Czech Republic was studied to investigate the biogeochemical response of A1 to high Ioadings of acidic deposition. The catchment supports Norway spruce plantations and is underlain by granite and podzolic soil. Atmospheric deposition to the site was characterized by high H + and SO42-fluxes in throughfall. The volume-weighted average concentration of total AI (Alt) was 28 lamol L -~ in the O horizon soil solution. About 50% of AIt in the O horizon was in the form of potentially-toxic inorganic monomeric A1 (Ala). In the E horizon, AIt increased to 71 ~tmol L -~, and AIi comprised 80% of AIt. The concentration of AI~ (120 p.mol L -t) and the fraction of AI~ (85%) increased in the lower mineral soil due to increases in AI~ and decreases in organic monomeric AI (Alo). Shallow ground water was less acidic and had lower AIt concentration (29 lamol L-~). The volume-weighted average concentration of A1 t was extremely high in stream water (60 lamol L -~) with AIi accounting for about 60% of AI t. The major species of AI~ in stream water were fluorocomplexes (AI-F) and aquo AI 3 § Soil solutions in the root zone were undersaturated with respect to all Alobearing mineral phases. However, stream water exhibited AI~ concentrations close to solubility with j urbanite. Acidic waters and elevated AI concentrations reflected the limited supply of basic cations on the soil exchange complex and slow weathering, which was unable to neutralize atmospheric inputs of strong acids.