The determination of experimental data on all processes that contribute to the radionuclide (RN) migration within the barriers of a deep geological radioactive waste repository is required to evaluate the long-term behavior of the waste repository. A novel approach for the experimental estimation of bentonite colloid diffusion coefficients in granite is proposed, since no experimental data on this issue are available. The Rutherford backscattering spectrometry (RBS) technique was selected, because it is suitable for the study and the detection of gradients of heavy elements in a well-defined matrix and allows measuring diffusion coefficients on a micrometer scale. To follow the trail of the bentonite clay colloids within the granite by RBS analysis the colloids were traced with a heavy element. Complementary batch sorption experiments guaranteed that the Eu was an adequate tracer. By RBS analysis, Eu concentration gradients within the granite were clearly observed, and it was then proven that traced bentonite colloids were able to diffuse within the granite. The bentonite colloids diffusion coefficient was estimated to be ≈10 -17 m 2/s, that is a value three orders of magnitude lower than that measured for the Eu as solute (10 -14 m 2/s).