Abstract: The invention relates to a process for mixing a polymer composition using an internal mixer having rotors 18, 20 rotatably mounted in a chamber 12, a ram 28 by which pressure can be applied to material in the chamber 12, and temperature control for the mixer. Materials are introduced into the chamber 12 and, after introduction of all the materials, at least three mixing variables e.g. temperature, time and energy, indicating mixing quality are monitored and control variables e.g. rotor speed and/or ram pressure adjusted dependent on the monitored mixing variables to achieve previously defined targets or if all are not simultaneously achievable, the most significant target. Control is initiated when the ram has reached a predetermined position and when at least one preselected condition e.g. time, temperature or energy, has been achieved.