Laboratory chloralkali membrane electrolyzer tests showed a dramatic voltage increase and mild catholyte foaming when low levels of chloromethyl triethylammonium chloride (CTACl), a quaternary ammonium salt, were present in the feed brine. Current efficiency was not measurably affected by CTACl. In contrast, laboratory membrane electrolyzers showed no voltage sensitivity to sodium gluconate, bisphenol A, or triethylamine, contaminants that are often present in interfacial polycarbonate plant byproduct brine. The voltage increase and onset of catholyte foaming were rapid when the feed was switched from ultrapure brine to CTACl-containing brine, requiring about 3 h to achieve a steady state. Both effects were completely reversible, but the system required about 20 h to return to baseline voltage after the feed was switched back to ultrapure brine. The cell voltage was remarkably sensitive to CTACl: 8 ppm CTACl yielded a 200 mV voltage increase vs ultrapure brine. Cyclic voltammetric measurements with CTACl-spiked brine showed no effect of CTACl on anode or cathode overpotentials. At steady state, 87% of the feed chloromethyl triethylammonium ion (CTA+) is recovered either in the electrolyzer catholyte as the hydroxide, CTAOH (56%), or in the depleted brine as CTACl (31%), which demonstrates that CTA+ is rather stable toward chloralkali conditions. It is concluded that the increased cell voltage is caused by chloromethyl triethylammonium ions adsorbing onto membrane ion exchange sites, which reduces the population of sites for sodium ion transport, and that catholyte foaming is caused by the presence of CTAOH in the catholyte. An adsorbent screening study showed that various carbons, including Ambersorb 572, are effective for CTACl removal from brine. A laboratory electrolyzer fed with Ambersorb 572 treated plant brine showed normal voltage and no catholyte foaming.