Polyphenols extracted from wood, brown leaves and green leaves of the River Red Gum (Eucalyptus camaldulensis DEHNH., Myrtaceae) were inhibitory towards purified alkaline phosphatises from porcine intestinal mucosa (PIMAP) and Escherichia coli billabongs. Brown-leaf extracts were generally more inhibitory than were green-leaf extracts, and polyphenols inhibited ECAP more than they did PIMAP. The threshold for inhibition of purified enzymes and most native enzymes ranged from <2-10 mgl -1, which was within the range of polyphenol concentrations commonly found in freshwater environments. Mixed-type inhibition and competitive-type inhibition were the most frequent effects of polyphenols on the purified alkaline phosphatises. Enzyme activity was increased after the extracts were treated with polyvinlpolypyrrolidone to remove polyphenols, suggesting that the extracts contained substances that activated alkaline phosphatases. The addition of Mg +2 (0.1mmol l -1) increased alkaline phosphatase activity, but this enhancement was progressively negated by the addition of polyphenols. In contrast, the addition of Zn +2 (0.1 mmol l-1) completely inhibited PIMAP activity, and again this inhibition was progressively negated by the addition of polyphenols. Polyphenols therefore will affect phosphorus regeneration in freshwaters not only by binding directly to the alkaline phosphatise enzyme, but also by complexing metals required by the enzyme for activity.