The effect of pH on the molecular size distribution of dissolved humic substances (DHS) in surface waters from four different lakes in northern Europe was studied using Sephadex G‐100 gel filtration chromatography (GFC). New methods for the analysis of the elution profiles obtained are introduced, employing parallel graphical and statistical approaches. For all samples investigated, the proportion of the DHS eluted in the excluded fraction tended to diminish with decreasing pH. At the same time, the remaining DHS eluted tended to decrease in apparent molecular size with decreasing pH. Analyses of the elution profiles revealed that the apparent conformational flexibility of the DHS in the samples was greatest for the samples of lowest ionic strength. The applicability and reliability of GFC as a tool for the investigation of aquatic humic substances are discussed.