We have designed a laboratory bioassay to expose fish to kinetically determined differences in aluminium hydroxide solution chemistry. We have used this system to demonstrate the hitherto unexpected result of an acute aluminium toxicity in the fish at a pH of 6.5. Supporting experiments have demonstrated that the mechanism of toxicity at this pH was probably an asphyxiation brought about by aluminium-induced changes in the rheological and diffusional properties of the mucus lining of the gill epithelium.