Gasification in supercritical water at pressures up to 30 MPa and temperatures around 600 degrees C is a very attractive method to use biomass with a high water content for energy production. Under these conditions hydrogen and carbon dioxide are formed as the main products. The reaction is significantly influenced by biomass ingredients. The effect of alkali salts is of special interest: the addition of alkali salts drastically increases the hydrogen yield, presumably as a consequence of the catalysis of the water-gas shift reaction. Furthermore, it is observed that in the presence of alkali salts backmixing has a positive effect on the gas yield. A possible explanation is that "active hydrogen" formed by the water-gas shift reaction inhibits the formation of low reactive species like coke and tarry compounds. The water-gas shift reaction is a late consecutive reaction, and this effect is evident if prior steps can be influenced by backmixing.