The peroxyacyl nitrates, a series of eye-irritating plant toxicants, are synthesized by gasphase reactions at low concentrations. They are purified by preparative-scale gas chromatography. The preparation and purification of the first member of the series by photolysis of ethyl nitrite vapor (ca 400 ppm) in oxygen is described in some detail. The entire procedure is designed to minimize factors which might cause decomposition of this extremely unstable product. Pure PAN is stored as a vapor diluted with dry nitrogen at about 100 psig and 40°F. Such mixtures (about 1000 ppm) can be analyzed by infrared spectrophotometry using a 10 cm cell. The product is used in studies of its toxicity toward biological materials, in instrument calibration, and for studies of air pollution chemistry.