To characterize the actual flow behavior of polymer, polystyrene (PS) and polypropylene (PP) macromolecular tracers were synthesized by copolymerization and graft routes. The two tracers were then used to obtain the residence time distribution (RTD) functions of PS and PP in a twin-screw extruder. The effect of temperature on RTD is discussed using different tracers. The mixing characteristics of different tracers in the extrusion of PS and PP were compared by measuring the overall and partial RTDs. The dependence of the overall and partial RTDs on the tracer types was investigated. The effects of mixing intensity, miscibility, and melting difference on the measured distribution were also discussed. POLYM. ENG. SCI., 54:310-316, 2014. (c) 2013 Society of Plastics Engineers