The adverse effect of natural organic matter (NOM) on the capacity of activated carbon to adsorb 2-methylisoborneol (MIB), a compound with an earthy/musty odor, is less severe for submicron-sized powdered activated carbon (SPAC) than for conventionally sized powdered activated carbon (PAC) [11]. In this study the NOM effect was confirmed, and the mechanism responsible for the effect was investigated by studies with another malodorous compound, geosmin. The mechanism was investigated with respect to the properties of NOM by simplified equivalent background compound (EBC) estimation and penetration index. Correlations between penetration index values and fractional areas of size-exclusion chromatogram indicated that higher NOM loading on SPAC were associated mainly with a fraction of NOM having a molecular weight (MW) >2 kDa and a chromophoric moiety, which did not diffuse into the inner region of adsorbent particles and instead adsorbed only onto their external surfaces. Therefore SPAC, which has a larger specific surface area per unit mass of adsorbent, adsorbs such high-MW chromophoric NOM to a greater extent than does PAC. However, such NOM does not compete for adsorption sites with geosmin because geosmin adsorbs onto the interior surfaces of adsorbent particles. Contrariwise, NOM with a MW of <2 kDa and with an nonchromophoric moiety penetrates adsorbent particles and adsorbs onto interior surfaces. The estimated EBC concentration and its correlations with both size-exclusion chromatogram fractions and penetration index values indicated the characteristics of the NOM that competes with geosmin to be similar to those of MIB. Chromophoric NOM with a MW of <230 Da competes for adsorption sites with both geosmin and MIB. Beside the nonchromophoric, low-MW (<2 kDa) NOM, such chromophoric, very-low-MW NOM also penetrates adsorbent particles and adsorbs onto interior surfaces. The loading of such NOM is therefore independent of the size of the carbon particles (SPAC or PAC). The NOM effects on geosmin adsorption capacity were therefore found to be similar for SPAC and PAC, despite the fact that more NOM was loaded onto SPAC than PAC. The very-low-MW chromophoric NOM accounted for <2% of the entire NOM. (C) 2013 Elsevier B.V. All rights reserved.