The valve comprises a rotary valve member, which serves to close the inlet port. A pivoted valve arm, which extends from the rotary valve member, and a pivoted link, which engages said valve arm, ensure that a valve disc moves in unison with the rotary valve member to a position in which the valve disc effects a pressure-tight closing of the outlet port. The valve is preferably mounted on a reactor for gasifying solid fuels under a pressure of 2 to 150 bars, particularly on the ash discharge side.