Following the eruption of Mount Pinatubo, and the subsequent transport of volcanic aerosol, significant depletions of stratospheric NO2 were observed at Lauder, New Zealand (45 ¿S,170 ¿E). The effect was more pronounced in NO2 columns measured at sunrise than those measured at sunset, and observed column amounts were reduced by more than 40% in October 1991. Concurrent ozone measurements obtained using the same technique were compared with Dobson measurements which confirmed that in October, apparent reductions of only 10--15% can be explained by geometric changes in scattering caused by the aerosols. We therefore conclude that a real depletion of NO2 has occurred. NO2 levels at twilight are dependent on many factors, but these observations may provide supporting evidence for the hypothesis that heterogeneous processes can convert N2O5 to HNO3 on the sulfate aerosol surfaces [Rodriguez et al., 1991>. If this proves to be the case it could, in the presence of current high chlorine levels and volcanically enhanced aerosol densities, lead to accelerated global ozone depletion. ¿American Geophysical Union 1992