Minerals, particularly clay-sized minerals, protect soil organic matter (SOM) from decomposition by microor-ganisms. Here we report the characterization of SOM andthe associated minerals over decades of biodegradation, ina French long-term bare fallow (LTBF) experiment startedin 1928. The amounts of carbon (C) and nitrogen (N) in thestudy area declined over time for six fractions (sand, coarsesilt, fine silt, coarse clays, intermediate clays, and fine clays).The C:N ratios of SOM associated with silt fractions re-mained constant, whereas the ratios significantly decreasedin clays, reaching very low values in intermediate and fineclays (C:N<5) after 8 decades of LTBF conditions. X-ray absorption spectroscopy revealed the following: (i) bulk-scale SOM chemical speciation remained almost constant;(ii) submicron particulate OM was present in coarse clays,even after 79 years of LTBF conditions; and (iii) illite par-ticles became progressively SOM-free with time, whereasmixed-layer illite/smectite and smectites were always associ-ated with OM throughout the bare fallow treatment. In sum-mary, these results suggest that clay-sized minerals prefer-entially protect N-rich SOM and that smectites and mixed-layer illite/smectite seem to protect associated OM more ef-fectively than pure illites.