Decomposition of ferrocenium by dioxygen in acetonitrile +0.1 M But(4)NClO(4) is reported. The nature of the products has been investigated by CV, FTIR, GC-MS, EQCM and elemental analysis. The insoluble materials, responsible for the commonly encountered electrode fouling during ferrocene oxidation in incompletely degassed solutions, is a mixture of hydrous iron(III) oxide and the organic polymer poly(cyclopentane-1,3-dione-4,5-diyl). Soluble products are mainly constituted by Fe(III) perchlorate, 4-cyclopenten-1,3-dione, tricyclo[5.1.2.0(2,6)]deca-4,8-dien-3,10-dione and related oligomers.