A decrease in surface tension with respect to pure water was observed in wet aerosol and cloud/fog samples. The measured decrease of surface tension is positively correlated with the concentration of total soluble organic compounds in the samples. On the basis of a previously developed methodology to fractionate soluble organic compounds into three different classes (neutral compounds, mono- and dicarboxylic acids and polycarboxylic acids), we investigated the surface-active behaviour of the compounds within each of these classes. Polycarboxylic acids having a molecular structure analogous to that of humic substances (humic-like substances) were found to be the most effective surface-active species within the droplets: three times more effective than mono- and dicarboxylic acids and one order of magnitude more than neutral compounds.