Cementing materials in the Arahama sand dune, Japan, were studied mineralogically and biogeochemically to gain a better understanding of the cause of hardening. The cementing material is a clay-organic complex composed of noncrystalline gels and a matrix of small, poorly crystalline particles showing 14–16-Å spacings. The gel materials appear to have transformed into the poorly crystalline particles, which have a high carbon content and Al/Si ratios of 2.2 to 2.0. These particles are slightly richer in Si and poorer in Fe than the gel materials themselves. The organic portion of the complex can be removed by H2O2 treatment, leaving a noncrystalline network-structure containing a dispersed granular component. Scanning auger-depth profiles of individual particles show a high surface concentration of C and O and an increase in the Al/Si ratio with depth. The energy-intensity distribution suggests a mixture of carbon compounds having a major core-line binding energy of a hydrocarbon. The gel-cementing materials in the sand dune may have been formed from biochemical weathering products of organic matter, which subsequently controlled the formation of clay-organic complexes.