In this study the fouling phenomenon of ceramic capillary filters in the filtration of humic acid was investigated by experimental work and by modelling. The fouling of humic acid on ceramic capillary filters was evident, and in most of the cases, irreversible. In some filtrations the fouling did not seem to be permanent, it looked more like a gel layer on the filter. The effect of pH was clear: the filtrate fluxes decreased to almost zero at pn rt and 8.5, but at pH 11 the flux reduction was only 50%. The structure of the humic acid molecule differs in alkaline and acidic conditions. Also, at pH 4 the charge of the filter medium is positive and the humic acid molecules are negative. The presence of ferric ions decreased the filtrate flux of humic acid faster than in the absence of them. The behaviour of humic acid was compared to other chelating agents. At ph 4 the difference between other chelating agents and humic acid was remarkable: tbe fluxes when filtering the others remained good during the filtration, but for humic acid the flux decreased very fast to close to zero. Elementary analysis showed that the inorganic elements did not adsorb on the filter surface. The fouling of humic acid fitted best to the standard blocking model, which means that the molecules fouled tbe filters by direct adsorption on the surface and the walls of the pores.