This work reports a carbon-nanotube amperometric sensor for the selective determination of 4-chloroaniline (4-CLA), the major degradation product found in commercial chlorhexidine solutions. The harmful 4-CLA is detected on a multi-walled carbon nanotube (MWCNT)-modified electrode, free from the interference of chlorhexidine and other substances present in such samples, with a detection limit of 50 nmol L−1. MWCNTs of two different dimensions were evaluated for 4-CLA detection and a 3-fold increase in sensitivity using both modified electrodes was obtained in comparison with the bare electrode. The association of batch-injection analysis with the amperometric detection provided high precision (1.2 %) and sample throughput (130 h−1). Analyses of samples by capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) attested the accuracy of the proposed method. Mouth rinse and skin disinfectant samples presented 4-CLA concentrations in the range between 4 and 235 μmol L−1.