The gas-phase reaction between the NO3 radical and peroxymethacrylic nitric anhydride (MPAN:(CH22C (CH3) C (O) O2NO2) has been studied by a relative-rate method at atmospheric pressure. A value of (1.45±0.45)× 10-16 c molecule-1 s-1 was obtained for the rate constant. The atmospheric chemistry of MPAN was considered in a diurnally varying model of the planetary boundary layer.