This article presents inelastic thermal neutron scattering experiments probing the phonon dispersion in mesoporous silicon with pores 8 nm across. Scattering studies reveal the energy–momentum relation for transverse and longitudinal phonons along the high symmetry directions ΓL¯, ΓK¯ and ΓX¯ in the Brillouin zone. The dispersion up to phonon energies of 35 meV unambiguously proves that the phonon group velocities in highly-crystalline silicon are not modified by nanostructuring down to sub-10 nanometer length scales. On these length scales, there is apparently no effect of structuring on the elastic moduli of mesoporous silicon. No evidence can be found for phonon-softening in topologically complex, geometrically disordered mesoporous silicon putting it in contrast to silicon nanotubes and nanoribbons. Display omitted •Inelastic neutron scattering shows phonon dispersion in mesoporous silicon up to the Brillouin zone boundary.•Stiffness tensor of crystalline porous silicon is determined.•No phonon softening in sub 10 nm structured porous silicon.