Abstract When liming running waters, dosers must compensate for different flow and water qualities and for the downstream inflow from acid tributaries which creates mixing zones. At a certain point in the mixing zone, a constant or fluctuating chemical disequilibrium will appear due to transformation processes. In laboratory assays, over-saturated solutions of aluminium with ongoing active precipitation of aluminium have been found to be especially toxic to fish. Recent experiments in a mixing zone in the limed River Audna, Norway, have confirmed this phenomenon. Atlantic salmon ( Salmo salar L.) and sea trout ( Salmo trutta L.) smolts were exposed to acid and limed waters and mixtures of the two waters downstream from the point of connection. In the acid tributary (mean values: pH = 4·8, Ca = 1·3 mg litre −1, Al i = 236 μg litre −1), LT 50 was 22 and 40 h for Atlantic salmon and sea trout, respectively. In the mixing zone (pH = 4·8–6·5, Ca = 1·2–3·2 mg litre −1, Al i = 50–240 μg litre −1), LT 50 was 7 h for both species, masking the normal species difference in tolerance. Osmoregulatory failure and rapid gill lesions occurred in the mixing zone as an effect of the transformation of Al into high molecular weight precipitating species. This is the first documentation of the existence of such highly toxic mixing zones in nature, and the results clearly show that the mixing zone is even more toxic to fish than acid aluminium-rich waters.