The rotational diffusion rates of molecules and of mobile molecule segments in crystalline solids are expressed as relaxation rates relative to the corresponding torsional oscillation rate of the molecule or molecule segment. The resulting dimensionless relaxation time τi(ΔHs/Ii)1/2turns out to be a function of packing density (*) of the crystal and of the characteristic temperaturefRT/ΔHs, wherefis the number of external degrees of freedom per molecule. The dimensionless activation energy ΔE/ΔHsis a uniform function of the packing density for crystals composed of rigid molecules and another function of * for crystals composed of flexible molecules. The rate determining factor for molecular rotation in crystals containing hydrogen bonds appears to be the breaking energy of the hydrogen bond.