Reaction between guanidinium nitrate and 18-crown-6 in ethanol yielded a crystalline complex of 2 : 1 stoicheiometry. Three-dimensional X-ray crystal structure analysis has shown the monoclinic unit cell, a= 8.969(1), b= 10.278(2), c= 14.253(1)à , β= 102.682(8)°, space group P21/c, to contain two centrosymmetrical entities, nitrateâ guanidiniumâ 18-crown-6-guanidiniumâ nitrate, the linkages being via a guanidinium hydrogen atom, H â ¦ O 1.91(13)à , to one oxygen atom of the ether, and from a second guanidinium hydrogen atom to one oxygen atom of the nitrate ion, H â ¦ O 2.08(18)à . Full-matrix least-squares refinement gave an R value of 0.083 for 711 reflections measured on a CAD-4 diffractometer. While the crystalline product isolated from an ethanol solution of guanidinium chloride and 18-crown-6 also has 2 : 1 stoicheiometry, calorimetric measurements establish that in methanol solution the interaction ratio is 1 : 1 even when the concentration of guanidinium chloride exceeds that of 18-crown-6 by a ratio of 4 : 1.