Structures of the complexes (1 and 8) of the guanidinium ion (H2N)(3)C+ with super Lewis acidic BH4+ and AlH4+ were calculated using the DFT method at the B3LYP/6-311+G** level. C-13 NMR chemical shifts were also calculated by the GIAO-MP2 method. Each of the dicationic complexes contains a hypercoordinate boron or aluminum atom with a two-electron three-center (2e-3c) bond. Guanidinium ion was found to form a strong complex with BH4+ but a relatively weak one with AlH4+. On the other hand, complexations of guanidinium ion with neutral BH3 and AlH3 lead only to very weak complexes (5 and 9). The structures of mono- and dicationic complexes were compared with the structures of protonated and methylated guanidinium dications.