Studies of self-diffusion in solids are presented, which are based on neutron reflectometry. For the application of this technique the samples under investigation are prepared in form of isotope heterostructures. These are nanometer sized thin films, which are chemically completely homogenous, but isotope modulated. Using this method, diffusion lengths in the order of 1 nm and below can be detected which allows to determine ultra low diffusivities in the order of 10-25 m2/s. For the model system amorphous silicon nitride we demonstrate how the structure of the isotope hetrostructures (triple layers or multilayers) influences the efficiency of diffusivity determination.