Twin screw extruder mixing of a typical black toner formulation was investigated under different mixing conditions. The intensity of the mixing was varied by changing the feed rate of the material and the screw rpm of the extruder. Microstructural changes in the particulate fillers were analyzed by transmission electron microscopy. The fillers were characterized in terms of their particle size and shape distributions and spatial distributions within the binder. Changes in the molecular structure of the binder polymer were identified by gel permeation chromatography and softening temperature measurements. Temperature-variable dielectric properties and rheological properties of the bulk material samples were investigated as possible tools for understanding the degree of dispersion. Results indicate that both methods are sensitive to the changes introduced by different processing conditions. Charging properties of the pulverized samples were also characterized. Both charge per unit mass values and charge distributions of the toner particles were found to be adversely influenced by increasing intensity of mixing.