A comparative study of the hypercoordinate square-pyramidal carbocations C7H9+ and C8H9+ was performed by the ab initio/GIAO-CCSD(T) method. The structures and C-13 NMR chemical shifts of the cations were calculated at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. The bishomo square pyramidal structure 1 was calculated for C7H9+ at the MP2/cc-pVTZ level. The calculated C-13 NMR chemical shifts of structure 1 agree extremely well with the experimental values. However, unlike for C7H9+ both the bishomo square pyramidal structure 3 and the trishomocyclopropenium type structure 4 were found to be minima on the potential energy surface of C8H9+. They are very close energetically with cation 3, only 0.7 kcal/mol less stable than cation 4 at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level. Neither structure 3 nor 4 yields NMR spectra that agree with experiment. However, a weighted average of the two reproduces the observed NMR spectrum of C8H9+ (at -80 degrees C) quite well.