Ammoniacal copper quaternary (ACQ) wood preservatives contain benzalkonium chloride (BAC) as the active ingredient. BACs in commercial ACQ products comprise mainly benzyldimethyldodecyl ammonium chloride (C12 homologue) and benzyldimethyltetradecyl ammonium chloride (C14 homologue). Previously, we developed solid-phase extraction (SPE) on Oasis MCX as a strong cation exchanger, followed by liquid chromatography with UV detection (LC-UV) for the quantitative determination of C12 and C14 homologues in treated wood. In this study, the previous method was modified. Samples were cleaned up before analysis, and a weak cation exchanger, Oasis WCX, was employed. C12 and C14 homologues were added to 500 mg of powdered wood samples of Japanese cedar ( Cryptomeria japonica ), Japanese larch ( Larix leptolepis ), Sakhalin fir ( Abies sachalinensis ), and Yezo spruce ( Picea jezoensis ) at levels of 10, 1, and 0.1 mg g -1 wood. Treated wood samples were then extracted with HCl/EtOH, and the extracts were filtered and subjected to SPE with Oasis MCX and Oasis WCX, followed by LC-UV analysis. Recovery rates ranging from 90% to 111% were achieved for SPE using both cation exchangers for active ingredient concentrations of 0.1–10 mg g -1 .