An apparatus for the transfer of solid, particulate materials from a first vessel (10) maintained at a relatively low pressure to a second vessel (30) maintained at a relatively higher pressure, which apparatus comprising a rotary valve (20) between the first and second vessels (10,30) wherein a vented annular space (24) is provided in a feeder section positioned between said first vessel (10) and said valve (20), said vented annular space (24) being positioned around a throat section (18) directly communicating between the outlet of said first vessel and the solid inlet of said rotary valve.