In the conversion of carbon dioxide to useful materials, the water–gas shift reaction is used to produce carbon monoxide from hydrogen and carbon dioxide. This is sometimes called the reverse water–gas shift reaction.
Water gas is defined as a fuel gas consisting mainly of carbon monoxide (CO) and hydrogen (H). The term ‘shift’ in water–gas shift means changing the water gas composition (CO:H) ratio.  The ratio can be increased by adding CO or reduced by adding steam to the reactor.