Visualisation studies of the dynamics of flow of rubber were undertaken using a laboratory-scale Banbury mixer with a transparent chamber. This technique was developed at the Institute of Polymer Technology, University of Technology, Loughborough under the guidance of Mr. P.K. Freakley. The method proved to ber very informative; the flow pattern could be recorded and its complexity identified. By measuring pressure and the rheologiccal properties of rubber, the stress distribution inside the mixing chamber could be calculated. From the flow patten and stress distribution, the modes of mixing inside the chamber could be identified as follows. Dispersive mixing occurs in the region at and immediately in front of the rotor tips while distributive mixing occurs at the S-shaped and bridge regions. There are also voids in the chamber where no mixing occurs, but the formation of these voids helps to enhance distributive mixing.