Fifteen English mine waters have been sampled and been subject to field determinations of alkalinity, pH and ‘cold’ acidity. Furthermore, acidity has been re-determined following stripping with compressed N2 gas and after boiling (‘hot’ field acidity). The difference between cold and hot acidity appears to reflect the temporary (CO2) acidity content of the water and is typically > 3 meq/l and reaches 15 meq/l in one case. N2 stripping is less effective at removing CO2 from the waters, although may represent a promising technique if employed for longer durations. While hot acidity is typically of the same order of magnitude as calculated permanent (metal plus proton) acidity, a poor degree of correlation is most probably due to a failure to implement the ‘hot peroxide’ acidity determination (incorporating neutralization of alkalinity and active peroxide oxidation of reduced metals, in addition to boiling) which is regarded as standard practice in the USA.