The effect of shearing in a corotating twin-screw extruder on the apparent kinetic rate of model reactions was studied. Oxazoline-terminated oligomers with different molar masses were prepared and used in condensation reactions with carboxylic acid terminated oligomers. A miscibility study of these oligomers allowed the choice of miscible and nonmiscible reactive systems. Reactions were first conducted in a glass reactor and then in the kneading-disc area of a corotating twin-screw extruder modified to neutralize the transport along the screws. For nonmiscible reactive systems, reactions were slower than equivalent reactions with miscible reactants. A shear increase for these nonmiscible reactants resulted in an increase in the apparent kinetic rate, which was in all cases lower than the rate obtained for miscible reactants. For very high shear rates, mechanical degradation was observed. (C) 2004 Wiley Periodicals, Inc.