The mixing hydrodynamics in a double planetary mixer is investigated numerically and experimentally over the course of a cross-linking reaction. Using various visualization techniques, it is shown that this mixer provides good radial dispersion capabilities but poor axial (top-to-bottom) pumping, irrespective of the viscosity level. The power drawn by the mixer evolves dramatically from about 180 W m(-3) at 40% conversion up to approximately 4 kW m(-3) at 85% conversion. Overall, the numerical predictions and the experimental results exhibit good agreement although at 85% conversion, the numerical model is not accurate enough to predict adequately the power consumption due to physical phenomena not considered in the computations.