Several nitrogen oxide compounds transform to nitrosonium nitrate (NO+NO3-) under high pressure. In this study, NO+NO3-was synthesized by laser heating of N2O in a diamond-anvil cell and characterized by x-ray diffraction as a function of pressure at room temperature and low temperature. The unit-cell parameters were refined up to 32.2 GPa at 300 K, resulting in a denser structure than reported previously. The pressure-volume relations for NO+NO3- at 300 K were fitted to both Birch-Murnaghan and Vinet equations of state. The analysis indicates that NO+NO3- is denser than other nitrogen-oxygen assemblages, consistent with the conclusion that formation of the ionic species is driven by density rather than entropic effects. The low-temperature x-ray-diffraction data represent the first in situ measurements for this material, revealing consistent structural information and evolutions under pressure. These observations provide additional information on the stability relations and reaction diagram of N2O and NO+NO3-. (C) 2003 American Institute of Physics.