The spectrum of magnetic resonance is typically affected by a modulated structure in an incommensurate phase, resulting in the broadened spectrum characterized by 'edge singularities'. We demonstrate that the modulated structure in the crystal where molecular motion exists is responsible for the narrowing of the spectrum in the fast motional region by means of the simulation of H-2 NMR spectrum of [Mg(H2O)(6)][SiF6]. The program developed by combining the theories of phase solitons in the incommensurate phase and motional change of H-2 NMR spectrum was used for the simulation. We obtained a good agreement between the theoretical H-2 NMR spectrum and experimental one for [Mg(H2O)(6)][SiF6]. (C) 2003 Published by Elsevier B.V.