Perhaps mankind's most urgent challenge at present is anthropogenic climate change, with the associated sea-level rise and desertification set to produce major losses of arable land and living space, as well as loss of life. The key to preventing the worst effects of AGW lies in limiting humanity's emissions of the greenhouse gas carbon dioxide, of which the vast majority comes from the burning of fossil fuels such as coal, oil and natural gas. However, fossil fuels are embedded in all of the world's economies, responsible for almost all of the provision of electrical power and transport, making the sizable reductions required in the timescale necessary somewhat impractical. One solution lies in Carbon Capture and Storage (CCS), which involves, in one incarnation, the combustion of fossil fuels in pure oxygen, simplifying the processing and storage of the carbon dioxide produced. There is potential for very high process efficiencies if oxygen is provided by Oxygen Transport Membranes (OTM). This thesis is concerned with the development of membranes and test procedures for mixed-phase OTM, which typically consist of a dense, gastight layer of perovskite and fluorite phases. An inactive support layer may also be present. The surface area, and therefore surface exchange of either side is improved by the addition of exchange layers to either side. Oxide ion migration is accomplished by applying a pO2 differential to the membrane at high temperature. Causes and mechanisms for degradation are not fully understood, and there is potential to improve oxygen flux. One way to achieve this is by the use of very thin, supported membranes, and this thesis demonstrates that such membranes can be fabricated with well-understood manufacturing processes. Another method of improving oxygen flux is by the use of catalysts on the exchange layers of the membrane. The most popular method of introducing 6 catalysts to an exchange layer or electrode involves impregnation of a metal salt into a ceramic backbone, followed by reduction to yield a catalytically active phase. However, this process is wasteful of catalyst, labour-intensive and control of the distribution of catalyst is difficult or impossible. An alternative exists, where metals doped into a perovskite migrate to the surface and form nanoparticles on exposure to a sufficiently high temperature and reducing atmosphere, and this thesis demonstrates the benefits of using such an approach. Improvements in oxygen flux of up to a factor of 7 over an undoped perovskite exchange layer have been demonstrated. The conductivity and crystal structures of (La0.8Sr0.2)0.95Cr0.5Fe0.5O3-δ and (Sc2O3)0.19(CrO2)0.01(ZrO2)0.789O1.94 under oxidising and reducing atmospheres at high temperatures have been evaluated using neutron powder diffraction and a novel in-situ rig, demonstrating that the OTM composition is a p-type conductor, and quantifying the effect of oxygen stoichiometry on conductivity and unit cell parameters.