The complexation of guanidinium perchlorate (I) with 18-33 ring atom crown ethers and with different subunits (e.g., catechol and 1,3-xylyl moieties) in examd. by 2-phase extns. The 18-crown ethers are able to form perching complexes; crown ethers with >=27 ring atoms have a ring size capable of forming encapsulated complexes with I. Arom., catechol, and esp. 1,3-xylyl moieties have a destabilizing effect on complex formation. The crystal and the mol. structures of the 1:1 complexes of I with benzo-27-crown-9, dibenzo-27-crown-9, and dibenzo-30-crown-10, are detd. by x-ray crystallog. In these encapsulated complexes all H atoms of the guanidinium cation are used in H-bonds to the macrocyclic host. The 27-crown ethers show an optimal fit between cation and the macrocyclic host with a complementary binding scheme. Dynamic 500-MHz 1H NMR gave the kinetic stabilities of these complexes with Delta Gd.thermod. values of 11.5, 11.2, and 12.0 kcal mol-1 for the complexes with benzo-27-crown-9, dibenzo-27-crown-9, and dibenzo-30-crown-10, resp.