The mechanism of non-catalysed and iron-catalysed water vapour gasification was studied with Polyvinylchloride cokes of different heat treatment temperature by temperature programmed desorption of carbon monoxide, from frozen, in surface complexes. Non-catalysed gasification is controlled by the formation of extremely stable ‘unreactive’ ethers formed at 500 °C. Dissociation of water at these surface complexes represents the rate limiting step. For iron-catalysed gasification, the oxygen transfer mechanism was confirmed. Three iron oxygen surface complexes of different stability may be formed, but steady-state isothermal gasification is only possible with the most stable complex as intermediate. Dissociation of water at the metal is accelerated and the oxygen transfer from the iron to the carbon surface controls the overall rate of gasification above 700 °C.