Anomalous NOx responses were observed when halocarbons were irradiated in the presence of oxides of nitrogen. Interferences to chemiluminescent NOx monitor using heated carbon converter were studied for phosgene, tri-chloroacetyl chloride, chloroform, chlorine, and hydrochloric acid. Data show positive interferences in the NOx monitor due to the test compound, either individually or as mixtures. Substituting an FeSO4 converter for the heated carbon converter may solve the problem. This information may be of particular interest to smog chamber researchers and to those making NO2 measurements in areas where halocarbon levels are relatively high. It is speculated that excited O2 molecules may be the interfering compound, but additional work is needed to substantiate this hypothesis.