Fe(III) ions may form ordered domains wherein the EPR resonance extends to the whole particle volume, leading to a major increase of spectral line intensities. This phenomenon of "strong exchange" is known as ferromagnetic resonance (fmr). The authors observed another type of exchange, a weak one, characterized by drastically reduced resonances. Interestingly, both exchanges run parallel (up- or downwards) with the catalytic activity of iron oxide nano-particles as catalysts in the N2O-mediated selective oxidation of benzene.