Results are presented from an experiment designed to investigate the deposition of Al species onto fish gills following the mixing of limed and acidic natural waters. The natural waters were labelled with 26Al and the distribution between high and low molecular weight forms was determined by ultrafiltration of water samples. In labelled acidic waters, the 26Al was present predominantly in low molecular weight forms, whereas in labelled limed waters the major fraction of 26Al was present in a high molecular weight form. In mixing experiments, 26Al was only detectable on fish gills when the 26Al was initially present in a low molecular weight form (i.e., in labelled acidic waters). Aluminium-26 was not detected on the gills of fish exposed to labelled limed waters. These results support the hypothesis that Al on fish gills arises from polymerization of low molecular weight species and that, within mixing zones, the high molecular weight species do not play a significant role in the precipitation of Al onto the gill surface.