The dynamics of a chiral polymorphic liquid crystal smectogen (8BEF5), partially deuterated on the aromatic core and on the chiral chain, has been investigated in its smectic A and C' phases. The H-2 T-1Q and T-1z spin-lattice relaxation times. and. consequently, the spectral densities J(1) (omega(o)) and J(2)(2omega(o)) have been determined for the aromatic as well as methylene deuterons of the chiral chain closer to the aromatic core. The interpretation of the relaxation behaviour has been carried out by means of the available motional models in the SmA phase. Contrary to what is usually observed for other mesogens, in the case of 8BEF5 the internal rotations are much better described by the strong collision model rather than by the small step rotational diffusion one. (C) 2002 Elsevier Science B.V. All rights reserved.