A pneumatic seat cushion system (10) has a deformable cushion (11) having a seating surface (12) and a self inflating cushion bladder (14) positioned beneath the seating surface (12). The self inflating cushion bladder (14) has an airtight envelope (15) containing compressible material (16) which is in air communication (19) with a displacement bladder (18) such that when in use the airtight envelope (15) and the displacement bladder (18) form a closed system from which no air can enter or escape. When there is no person seated on the seating surface (12), the self inflating cushion bladder (14) is inflated and the displacement bladder (18) is deflated. When there is a person seated on the seating surface, the weight of the person compresses the compressible material (16) so as to partially deflate the self inflating cushion bladder (14) which results in the contained air passing into the displacement bladder (18) which becomes fully inflated.