Humin is the fraction of the humic materials that is insoluble in an aqueous solution at any pH value. Its nature is complex and consists of humic, lipid, and mineral components. It is perhaps best represented as an organo-mineral composite. It displays a significant aliphatic character that is probably attributable to the lipids that comprise a significant portion of its organic components. The humic component of humin consists of aromatic and carbohydrate carbon with an aliphatic carbon content that is less significant. The organization of these components in the composite is unknown, but characterization of humin using small-angle x-ray scattering has shown it to be a mass fractal material in which the organic components are responsible for the particle surface roughness. Monte Carlo simulations of the organization of the components of humin are consistent with a variety of sorption and spectroscopic experiments that suggest that different organic matter “domains” exist in humin as they are postulated to exist in whole soil or sediment organic matter. Studies using carbon-14 labeled anthropogenic organic compounds (AOCs) such as pesticides, herbicides, PAHs and PCBs have shown that most of the radioactivity applied to a soil is bound to humin. When humin is fractionated with the MIBK partitioning method, the largest portion of the bound radioactivity for more highly functionalized AOC is associated with the humic acid-like component of humin; more hydrophobic organic compounds tend to be associated with the lipid fractions. Insights into the nature of humin gained from its interactions with AOC have lead to evolving understanding of the “architecture” of this composite.