Interactions between benzalkonium chloride (BAK) and the respective polymers, and the related availability of BAK in the corresponding solution, were studied for aqueous preparations of hydroxyethylcellulose (HEC), polyvinyl alcohol (PVA) and crosslinked poly(acrylic acid) (PAA). The study was performed by means of a crossflow filtration process with an alumina membrane. In the presence of the PAA gel, the rejection rate of BAK is much higher than with HEC or PVA. These results can be explained by the association of the benzalkonium cation BAK(+) with the negative carboxylate groups or the PAA. This effect : which was confirmed by dialysis experiments, leads to the trapping of BAK(+) inside the polymer network and could be of interest in the reduction of the harmful effects of BAK observed in the treatment of eye diseases.