A thorough investigation on the denuder method for ambient NO2determination, based on the reaction with phenoxide ions has been made. A thermostatted sampling unit comprising an annular denuder coated with alkaline eugenol and a nylon pre-filter has been found to be suitable for ensuring quantitative sample collection and preventing interferences from nitrous acid as well as from gaseous and paniculate nitrate. Results of laboratory and field tests concerning the effect of humidity on collection efficiency, the oxidation of nitrite on the wall by O3/oxidants and the reliability of the method are reported and discussed.