The molecular weight (M-w) fractionation of purified Aldrich humic acid (PAHA) resulting from adsorption on kaolinite and hematite was investigated for different solution pH and phosphate conditions. Adsorption was highly pH-dependent, with higher uptake at lower pH values. For all pH conditions, the weight-average MW (MWw) of residual PAHA remaining in solution after adsorption deviated from the original M-w, indicating that preferential adsorption of certain M-w components occurred. The extent of preferential adsorption depended on the percent carbon adsorption at a given pH condition. For similar percent carbon adsorption ranges, a greater extent of preferential adsorption of the higher M-w PAHA components was observed with higher pH values as demonstrated by the lowest residual MWw value occurring at pH 9. Detailed analyses of selected residual PAHA samples clearly showed that adsorption selectivity for particular M-w components was strongly influenced by solution pH. The extent of preferential adsorption of lower M-w PARA components decreased in the presence of a small amount of phosphate. This effect was more evident for hematite than kaolinite, and became greater with lower solution pH irrespective of the mineral type. The different fractionation patterns observed for PAHA were reasonably well explained by the physicochemical trends occurring in its MWw fractions and the underlying sorption processes. (C) 2004 Elsevier Inc. All rights reserved.