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The effects of Sn doping on the superconductivity and the electronic properties of the YBaSrCu3−xSnxOy system have been investigated by electronic transport and electron paramagnetic resonance (EPR) measurements. The superconductivity of the system is gradually depressed by Sn doping. Meanwhile, the carrier concentration is decreased and the EPR absorption is restored by doping Sn4+ at the Cu(1) site. Our results strongly suggest the localisation of the Cu2+ states at Cu---O chains, which is responsible for the reduction of the carriers and the suppression of the superconductivity.