The elemental and ionic composition of the solar wind is determined very close to the Sun. Composition data therefore contain direct information about the structure of the corona and the processes which lead to the generation and acceleration of solar wind and can constrain processes occurring in the heliosphere. We use Advanced Composition Explorer (ACE)- Solar Wind Ion Composition Spectrometer (SWICS) composition measurements of the solar wind to examine first the structure of the slow solar wind. We find that the statistical properties of the data suggest the presence of a large number of composition boundaries, where compositional patterns change abruptly. These discontinuities are interpreted here as a consequence of the structure of the solar wind source region. Using the experimental constraints of these observed composition boundaries, we derive direct constraints for turbulent diffusion of magnetic field lines in the heliosphere. We then discuss consequences for the propagation of energetic particles. ¿ 2000 American Geophysical Union