A reactor including a rotatable disc (3) having a surface (5) onto which reactant (15) is supplied by way of a feed (4). The disc (3) is rotated at high speed, and the reactant (15) spills over the surface (5) so as to form a film (17). A shear member (18, 20) is mounted close to the surface (5) so as to contact the film (17) during operation of the reactor, thereby applying a shearing force to the reactant (15) so as to aid mixing.