Ethylene/oxides of nitrogen mixtures were irradiated in the absence and presence of diethylhydroxylamine. As previously reported, the presence of diethylhydroxylamine inhibited the photooxidation of the hydrocarbon and nitric oxide and the onset of ozone formation. Once the diethylhydroxylamine completely reacted, the ozone rose more rapidly, but to a lower level than in the absence of diethylhydroxylamine. Peroxyacetyl nitrate was also produced with the addition of diethylhydroxylamine. The reaction mixtures were tested for total mutagenic activity by gas phase exposure to Salmonella typhimurium strain TA100. A significantly greater mutagenic activity was observed in the irradiated ethylene/diethylhydroxylamine/oxides of nitrogen mixture relative to the irradiated ethylene/oxides of nitrogen mixture. Only approx. 20% of the observed response could be accounted for by known reaction products.