The contribution of lipids and proteins to the surfactant properties of humic acids extracted from six different soils was examined. The variation of surface tension with humic acid concentration at pH 9 was analyzed for the untreated, lipid extracted and protein extracted humic acid. The results showed that the removal of lipids significantly decreases the surfactant behavior of the humic acid. Upon lipid removal the humic acid behaves as a weaker surfactant, lowering the surface tension of water with 1–5 % and exhibits micelle formation at higher concentration. Removal of protein from soil by citrate extraction determines values of surface tension higher than the untreated humic acid and lower than the lipid extracted samples. For some of the samples the values are close or identical to the untreated samples, while others are closer to the values for the lipid-extracted samples. The implications of these findings on the understanding of the structure of humic substances are discussed.