Over the past two decades, acidification has been one of the central issues in environmental research. The subproject of the Humic Lake Acidification Experiment (HUMEX), begun in 1988, was designed to determine the contribution of organic acids to the acidity of Lake Skjervatjern (the HUMEX Lake) and the impacts of the acidification on the characteristics of organic carbon. In order to get an empirical measure for organic acidity, dissolved organic carbon (DOC) was fractionated, isolated, and base-titrated from each half of Lake Skjervatjern. Hydrophobic acids were the dominant organic carbon fraction; the total organic acid content was generally greater than 80% of the DOC. The reliability of the fractionation procedures was tested with synthetic acids and the Nordic Fulvic acid. The DOC fractions did not show high variation over the 1.5-yr acidification period. Hydrophilic acids had consistently greater exchange acidities compared to hydrophobic acids. The dissociation of organic acids during acid-base titrations clearly increased with increasing pH. The high organic anion contribution to the ion balances indicates that humic matter is an important acidity source in Lake Skjervatjern.