A narrow singlet has been observed in the ESR spectra after benzene adsorption on sulfated zirconia, with its evacuation and freezing leading to the reversible formation of benzene dimer radical cations. The singlet has been proposed to result from the delocalization of the electron density throughout several benzene rings due to fast electron exchange. This distinguishes sulfated zirconia catalysts from zeolites where this effect is not observed because of the small size of channels.