We report on the interference arising in measurements of nitrogen oxides (NO_x) from the presence of a range of alkenes in sampled air when using the most widespread air quality monitoring technique for chemiluminescence detection. Interferences of up to 11 % are reported, depending upon the alkene present and conditions used. Such interferences may be of substantial importance for the interpretation of ambient NO_x data, particularly for high volatile organic compound and low NO_x environments.