Magnetic resonance imaging (MRI) has been used to determine quantitatively one-dimensional proton concentration profiles in clay gels prepared from water/heavy water mixtures, taking into account the variations of spin-spin relaxation rate and other effects on the signal intensity. The method has been used to determine the diffusion coefficient of water in Laponite gels at macroscale, by following the diffusion of water from a volume of gel prepared with water into a volume of gel prepared with heavy water: The results have been compared with the self-diffusion coefficients at microscale obtained by pulsed gradient spin-echo (PGSE) NMR. Both results are in agreement in spite of the complex structure of the gel, but the reduction in diffusion coefficient as compared to bulk water is larger than predicted from effective medium theories.