We present a method which allows quasi-elastic neutron scattering (QENS) at multi-GPa pressures, i.e., pressure conditions which are an order of magnitude higher than possible with QENS till now. The technique uses an opposed-anvil geometry with a sample chamber in the form of a toroid which minimizes multiple scattering effects. This method has been applied to measurements of the self-diffusion coefficient of water at 400K up to 3 GPa. The results demonstrate the potential of this method for studying diffusion over a large pressure and temperature range. (C) 2013 AIP Publishing LLC.