To understand the performance of multicomponent reactions in extruders, the mixing mechanism in the extruder has to be understood. Viscosity and quantity differences between the components fed to the extruder considerably influence this mixing process. The extent of their influence on the mixing and therewith the reaction process was investigated with a decolorization reaction in a fully intermeshing counterrotating twin screw extruder with a transparent barrel. It could be concluded that for this type of extruder faster mixing is achieved if one component (mostly a side feed) consists of low viscous material and is present in considerable amounts. Thus an increase in the viscosity ratio between both components leads to a decrease of the mixing length and an increase of the quantity ratio leads to an increase of the mixing length. The experimental results agreed well with the theoretical analysis of the flow behavior in the mixing areas of the extruder.