We use H-1 NMR relaxation measurements in a broad range of frequencies to extract information on the dynamical properties of water molecules in dilute suspensions of swelling clay particles. The frequency variation of the longitudinal relaxation rate suggests the existence of a 2D diffusion mechanism for water molecules adsorbed on the clay surface, with a residence time longer than 10(-6) s. This observation explains the paradoxical results frequently reported from NMR relaxation studies on such heterogeneous systems.