The low-frequency dynamics of different solid phases of ethanol ~fully ordered crystal, orientational glass, and true structural glass! is investigated by measurements of low-temperature specific heat as well as determination of the vibrational densities of states by inelastic neutron scattering. Such polymorphism provides a unique benchmark to study the relative importance of different kinds of disorder on glass-dynamics properties. The measurements are carried out both for hydrogenous and fully-deuterated samples. Large isotopic effects are found for the excess contributions ~i.e., non-Debye! to the specific heat of the disordered solids, which have clear correlates in the low-frequency spectra. @S0163-1829~98!05626-4#