Measurements of the reaction probability (&ggr;) of N2O3 on ~60 wt % H2SO4 were performed at 201 to 230 K on bulk liquids and at 230 K on aerosol particles. The bulk and aerosol results are in good agreement (less than 10% difference). The measurements on bulk liquids show very little change in &ggr; with temperature from 201 to 230 K. HNO3 was added to the bulk liquids at ~201 K and to aerosol particles at 230 K (the H2SO4-content was decreased to keep water activity approximately constant). For [HNO3> levels of 1-to-15 wt %, the N2O5 reaction probability decreased by ~15 % from that measured on HNO3-free 60 wt % H2SO4. For 60 wt % H2SO4 aerosol in the stratosphere ([HNO3>~0.3 wt %) the decrease in the N2O5 hydrolysis reaction probability due to HNO3 would be small.¿ 1997 American Geophysical Union