A reactor including a rotatable disc (3) having a region (13) in an upper surface (5) thereof. Reactant (15) is supplied to the region (13) by way of a feed (4), the disc (3) is rotated at high speed, and the reactant (15) moves from the region (13) so as to form a film (17) on the surface (5). As the reactant (15) traverses the surface (5) of the disc (3), it undergoes chemical or physical processes before being thrown from the periphery of the disc (3) into collector means (7).