The paper describes a hydrogen chloride detector designed to monitor concentrations of hydrogen chloride gas in the ambient environment. The detector was developed for NASA for use in launch vehicle effluent monitoring. The detector operates on chemiluminescence principles with a lower detection limit of less than 5×10−3 ppm (by volume). The hydrogen chloride in the air sample reacts with a bromide–bromate coating in the inlet tube of the instrument producing bromine. Bromine is then quantitated by chemiluminescent oxidation of luminol. The visible light generated in the chemiluminescent reaction is proportional to the hydrogen chloride concentration of the sampled airstream. The detector’s response to 90% of signal ranges from less than 1 s at 50 ppm to 10 s at 1 ppm. Below about 5×10−2 ppm the response is somewhat slower, of the order of several minutes. Due to the lack of specificity, the detector is most suited to laboratory or field studies where hydrogen chloride is the dominant pollutant, as compared to the interfering species. Interferences include strong acids, acid‐forming gases, and halogen gases. Of the interferences investigated the most serious in these groups are hydrochloric and sulfuric acid, sulfur dioxide, and chlorine, respectively. The detector has been in use since 1974 and has been found to be highly portable, rugged, and stable under extreme environmental conditions ranging from aircraft and seacraft operations to desert operations at temperatures above 35 °C.