We present a template-free synthesis of Fe3O4/SiOC(H) nanocomposites with in situ formed Fe3O4 nanoparticles with a size of about 50 nm embedded in a nanoporous SiOC(H) matrix obtained via a polymer-derived ceramic route. Firstly, a single-source precursor (SSP) was synthesized by the reaction of allylhydridopolycarbosilane (AHPCS) with Fe-acetylacetonate [Fe(acac)(3)] at 140 degrees C. The SSP was heat-treated at 170 degrees C to generate Fe3O4 nanocrystals in the cross-linked polymeric matrix. Subsequently, the SSP was pyrolyzed at 600 degrees C-700 degrees C in argon atmosphere to yield porous Fe3O4/SiOC(H) nanocomposites with the high BET surface area up to 390 m(2)/g, a high micropore surface area of 301 m(2)/g, and a high micropore volume of 0.142 cm(3)/g. The Fe-free SiOC(H) ceramic matrix derived from original AHPCS is nonporous. The in situ formation of Fe3O4 nanoparticles embedded homogeneously within a nanoporous SiOC(H) matrix shows significantly enhanced catalytic degradation of xylene orange in aqueous solution with H2O2 as oxidant as compared with pure commercial Fe3O4 nanoparticles.