We recently observed a resonant spin excitation in Ba0.6K0.4Fe2As2, one of the newly discovered iron arsenide superconductors with Tc=38K, on MERLIN. This demonstrates that the sign of the energy gap is opposite on the disconnected hole and electron Fermi surfaces at the Brillouin zone centre and zone boundary respectively, indicating that the pairing potential is repulsive at short range. The energy of the excitation scales with the maximum superconducting energy gap in the same way as similar excitations in high-Tc copper oxide and heavy fermion superconductors. In this proposal, we plan to study the Q- and temperature-dependence of this excitation in Ba0.8K0.2Fe2As2, which is at the transition between spin-density-wave order and superconductivity.