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Catchments that contain acid sulfate soils can discharge large quantities of acid and dissolved metals into waterways. At McLeods Creek in far northern NSW, Australia, the acidity from the hydrolysis of dissolved metal species, particularly aluminium and iron, contributes to greater than 70% of the total acidity. Therefore, a poor relationship exists between both calculated and titrated acidity and pH because of the dominant influence of these hydrolyzable metal species. Determination of the so-called `cold acidity` by direct titration with NaOH yields results that are difficult to replicate because of the buffering effects of suspended solids, carbon dioxide ingassing, and/or Mn-II and Fe-II oxidation in the sample as the titration end-point is approached. Samples that are pre-treated with sulfuric acid and hydrogen peroxide produce results ( of `hot acidity`) that can be easily replicated and are similar to calculated acidities based on elemental analysis and speciation calculations. The cold acidity values for titrations of 105 water samples from the chosen field site are often higher than hot acidity values as a result of the loss of carbonate acidity during pre-treatment of samples for hot acidity analysis.