Low-rank coal derived humic acids were fractionated by ultrafiltration to investigate the effects of various experimental conditions on their apparent ＇molecular weights＇. With decreasing pH the apparent molecular weight of the humic acids increased dramatically. Aggregation seems to be the reason for this effect. However, detergents, organic solvents or EDTA had a negligible effect on the fragmentation pattern. Oxidation of the coal with H2O2 before the extraction of the humic acids and also the alkaline strength during the extraction influenced the molecular weight of the extracted humic acids to a remarkable degree. Fractionation obtained by ultrafiltration was compared with data of size exclusion chromatography as another technique to determine the molecular weight distribution.