Polymer blending is a very important polymer processing operation. It aims at preparing new polymer materials by blending existing polymers without the need of creating new molecules. For immiscible polymer blends, it is always challenging, if not impossible, to choose an appropriate compatibilizer and assess its compatibilizing efficiency under pilot or industrial polymer-blending conditions. The concept of compatibilizer-tracer developed in this work is able to take this challenge. This is shown using polystyrene (PS)/polyamide 6 (PA6) blends and fluorescent labeled graft copolymers of PS and PA6 as compatibilizer-tracer. Transient experiments allow using very small amounts of compatibilizer-tracer to obtain emulsification curves, namely, the evolution of the dispersed phase domain size as a function of the compatibilizer-tracer concentration. Other potential applications of this concept are discussed and its limitations are investigated. (C) 2010 American Institute of Chemical Engineers AIChE J, 58: 19211928, 2012