Antiseptic agent carbethopendecinium bromide (septonex) was determined by capillary electrophoresis with capacitively coupled contactless conductivity detection. Optimal separation of this quaternary ammonium ion was achieved in BGE of pH 7.0 containing 30 mM 2-(N-morpholino)ethanesulfonic acid, 12.5 mg/mL of 2-hydroxypropyl-β-cyclodextrin and 20% v/v of acetonitrile. The separation was performed at 25°C in an uncoated fused silica capillary (50 μm id; total length, 60.5 cm; effective length, 50 cm) at 30 kV. Samples were injected hydrodynamically at 50 mbar for 6 s. For quantitative analysis, L-arginine (500 μg/mL) was used as internal standard. The calibration curve was rectilinear for 25-400 μg/mL of septonex (y=0.0113x-0.0063; r(2)=0.9992). The LOD was 7 μg/mL of septonex (at S/N=3). The run-to-run repeatability (n=6) was characterized by the RSDs of 0.18% for the migration time and 1.96% for the analyte/internal standard peak area ratio. Accuracy tested by recovery experiments at three concentration levels gave recoveries of 100.27-104.22% with RSD ≤2.19%. The method was successfully applied to the assay of carbethopendecinium bromide in eye drops. Quaternary ammonium ions having structure and size close to that of carbethopendecinium may not be resolved from the analyte.