The effects of particle size and mixing method of adsorbent during pretreatment in a microfiltration system were investigated. Micrometre-size powdered activated carbons (PACs) obtained by pulverization of an as-received PAC removed natural organic matter (NOM) from water much more efficiently than the as-received PAC, without additional membrane fouling and trans-membrane pressure buildup. The dosage of micrometre-size PAC required for pretreatment was one-sixth the equipollent dosage of the as-received PAC. The coagulant – PAC dosing sequence did not affect the extent of NOM removal, suggesting that the reduction of PAC-pore-blocking macromolecules by coagulation before PAC adsorption was not a key factor in improving the final extent of NOM removal. The mechanism of the enhanced adsorption by micrometre-size PAC seems to involve increases in the adsorption capacity itself, as well as increases in adsorption kinetics.