In 50 lakes in south-eastern Norway hydrogen-ion concentration (pH) and total hardness (°dH) were measured in surface water in the 1950's and then once again 10 years later. Significant acidification was observed. In the total material average pH dropped from 7.0 to 6.8. The lakes were grouped into a low-total-hardness (0-l°dH) and a high-total-hardness group (>l°dH). [l°dH = 10 mg »CaO«/l]. In the low-total-hardness group average pH had dropped from 6.6 to 6.3 and the average H+ concentration increased with 4.33. 10−7 mol/1. In the high-total-hardness group significant acidification could hardly be traced. The data confirm previous observations that also lakes which in previous years had pH close to the neutral point have become acidified. Total hardness showed no significant change.

Acidification seems to occur most frequently in lakes with total hardness ⩽l°dH. Such lakes are present all over Norway and are dominant in many areas.

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