On-line theological measurements are usually employed to monitor the reactive modification of single polymers during extrusion. This article provides data on the use of an on-line rheometer to monitor the reactive compatibilization of polyamide/polyolefin blends in a corotating twin-screw extruder. The type of functionalized compatibilizer and the choice of compounding conditions are among the variables examined. On-line and off-line theological data are correlated with the morphologies of the extrudates and the mechanical properties of injection-molded samples in attempts to establish general relationships between on-line theology, microstructure, and properties of polymer blends.