A fluidized bed reactor for the gasification of coal dust with super- heated water under pressure comprises a reactor vessel (1) having an inlet for water vapour (3) in its base, an inlet (5) and an outlet (6) for maintaining a bed of coal dust, an outlet (11) for product gas, a heating tube coil (10) extending into the bed, and at least one wall (13) located in the bed (14) to impede direct flow of material from the inlet (5) to the outlet (6); the material is thus forced to follow a tortuous route, e.g. as shown by the arrows (15). The wall may be spiral in form.