The amount of hydrogen adsorbed in arrays of single-walled carbon nanotubes has been studied as a function of the geometry of the array. The tube lattice spacing has been varied to optimize hydrogen uptake. Two different lattice geometries have been examined, namely, the triangular lattice and the square lattice. None of the geometries studied are capable of achieving adequate hydrogen storage capacity for use in vehicular fuel cells at room temperature. The strength of the solid-fluid interaction potential has been increased in order to identify a combination of potential and geometry that will meet the DOE targets for hydrogen storage for fuel cell vehicles. The DOE target values cannot be reached even by tripling the fluid-wall potential at ambient temperature. However, it is possible to achieve the DOE targets at a temperature of 77 K, but only if the strength of the interaction potential is increased by about a factor of 2 and the lattice spacing of the tubes is optimized. On the basis of these observations, carbon nanotubes do not appear to be useful adsorbents for vehicular hydrogen storage applications.