Small-angle neutron scattering (SANS) and diffusion NMR studies are performed to investigate the stability and geometry of hydrogen-bonded pyrene-guest-containing C-hexylpyrogallolarene (PgC6-pyrene) nanotubular frameworks in solution. In the solid state, hydrogen-bonded pyrogallolarene tubes are formed; however, the scattering data for PgC6-pyrene assemblies in acetone are best modeled as dimeric spheres of PgC6 with no pyrene guest. The result of diffusion NMR study also indicates the rearrangement of tubular entity into spherical framework in acetone. This is the first example of structural transformation of pyrogallolarene nanotubes (guest-exo) in solution. Individual hydrogen-bonded spheres of PgC6 exhibits a uniform radius of ca. 8.6 Å and a diffusion coefficient of 9.12 × 10−10 m2 s−1 in acetone. The diffusion NMR measurements further gave, for the first time, insights into how the type of solvent (acetone vs. methanol vs. acetontitrile/D2O) governs the structural differences in these nanoassemblies. Solution-phase structural alteration observed for PgC6-pyrene gives evidence of enhanced stability of pyrogallolarene nanocapsules over nanotubes.