Trigonal crystals of monoguanidinium dioxonium trinitrate have been synthesized and investigated as a function of temperature by X-ray diffraction and by DTA. The strongly anisotropic crystals, built of hydrogen-bonded corrugated sheets with clearly pyramidal H3O+ cations, are stable below 280 K, and show an onset of negative thermal expansion along the hydrogen-bonded sheets at 140 It. analogous to that in H2O ice Ih below 80 K. We have also detected a similar negative expansion region about 80 K in guanidinium nitrate, [C(NH2)(3)](+). NO3-, measured by neutron powder diffraction between 4 and 160 K. Factors destabilizing the [C(NH2)(3)](+). 2H(3)O(+) 3NO(3)(-) Structure above 280 K have been discussed, and the role of the dynamics of the H3O+ cations postulated.