AFTER the excursion to the limiting case of apparent sedimentation coeffi-cient distribution of non-diffusing particles, we focus back on trying to find a model that aims to embody the true physical process of sedimentation and the real properties of the sedimenting particles, and to stringently test the model by fitting to the complete set of available experimental data. Having laid the groundwork of defining and examining the characteristics of distributions and discussing the computation of distributions by direct least-squares fitting of the raw sedimentation data over the time course of the whole experiment, we can advance to the distributions of diffusing particles simply by exchanging the kernel of the integral equations from the step functions χnd(s, r, t) of non-diffusing species to the Lamm equation solutions of real, diffusing species, χ(s,D, r, t).