Means of minimising carbon monoxide in a hydrogen stream for fuel cell operation are reviewed. Reduction of carbon monoxide to an acceptable level of 10-50 ppm involves high temperature and low temperature water gas shift, followed by selective oxidation of residual carbon monoxide. Methanation of very small amounts of carbon monoxide may be an alternative final step. A new range of promoted iron-chromium catalysts for high temperature water gas shift is shown to proceed via a redox mechanism. Precious metals, copper and gold catalysts, mainly supported on ceria, are efficient for the low temperature reaction: ionic gold, held within an oxide lattice, is shown to be particularly effective, even in the absence of nanostructured gold particles. Selective oxidation is also promoted by gold-based catalysts but precious Metal Systems are widely used. The additional cost associated with the use of two reactors-to maintain temperature control-drives research into alternative systems. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.