Abstract Organic acid anion concentrations ( R −) represent an important part of the anion pool in colored surface waters. When the estimation of R − is based on a charge balance approach, R − cannot be used for the ionic balance control of the correctness of the analyses. We provide here another simple approach for R − estimation, which is independent of the charge balance. In this approach, R − was estimated using the concentration of ionizable carboxylic groups per mass of DOC ( X I), DOC concentration and pH. The X I value was estimated as the difference between the average total concentration of carboxylic groups per 1 mg of DOC of natural fulvic and humic acids (10 μeq mg −1) and the concentration of metal-complexed carboxylic groups. The latter concentration was assumed to be equal to organically-bound Al and Fe. When applied to the Bohemian Forest stream waters, the average (±standard deviation) X I was 6.2±0.8 μeq mg −1, R − varied from 2 to 146 μeq l −1, and the average percent difference between the sums of cations and anions was 0.7%. The R − values, obtained using the charge balance approach, were strictly comparable ( P<0.001) to the values estimated from the proposed approach.