Rotor-stator mixers are commonly used in energy intensive processes but there is relatively little published information on which to base process design. This study investigated the de agglomeration of nanoparticle clusters in a liquid to determine the effects of solids loading (up to 15%wt) and continuous phase viscosity (up to 100 mPa s) on the mechanisms and kinetics of breakup and dispersion fineness in an in-line rotor-stator. A Silverson 150/250MS rotor-stator was used in the recirculation loop of a stirred tank. It was shown that the power number values previously obtained at Reynolds numbers greater than 200,000 are constant at Reynolds numbers as low as 2400. It was found that the breakup kinetics were not significantly affected by the solids loading, within the range covered in this study. On the other hand, when the viscosity of the continuous phase was increased, the de-agglomeration became slower even though the solids concentration was low (1%wt) and the flow through the rotor-stator was still turbulent. This indicates that it is the flow conditions around the particle and not the bulk rheology of the dispersion that determines the kinetics of the de-agglomeration process. Breakup mechanism was found to be erosion and the dispersion fineness was determined by the size of aggregates. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.