Distributive mixing was experimentally measured during polymer melt blending along the length of a co-rotating twin-screw extruder. A mixing limited interfacial reaction between two reactive polymer tracers was employed to gain direct evidence of the generation of interfacial area. Model reactions were performed to validate this novel experimental method. In particular, the conversion was a direct indicator of the interfacial area available for the reaction. Specially designed sampling ports were used to obtain polymer samples along the length of the extruder during its continuous operation. The mixing capabilities of conveying sections and kneading blocks were compared over a wide range of operating conditions. In conveying sections, distributive mixing was controlled by the fully filled fraction. The mixing in kneading blocks depended on the combination of the operating conditions and the stagger angle.