Angle resolved 2-D PIV measurements were taken of the flow inside an in-line Silver son 150/250 high shear mixer, facilitated by a custom built transparent poly methyl methacrylate mixing head. Water was used as the working fluid in the turbulent regime (120,000 < Re < 420,000) and measurements were taken in a field of view encompassing one quarter of the whole mixing head. The effect of the presence of an additional externally driven flow from a pump, and back pressure using an outlet control valve, were examined. The results show that the dominant flow pattern can either be radial or tangential with the former being driven by the external flow at high flow rates and the latter at increasing rotor speeds. Increasing the back pressure promoted tangential flow. Detailed measurements were made of the influence of these parameters upon the mixer flow pattern and calculation of flow number from 2-D Ply data; the latter are strongly influenced by the geometry of the mixer and pipework. Crown Copyright (C) 2018 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.