A novel in-line melt rheometer (ILR) measured viscosity to control ionomer neutralization during reactive extrusion. The ILR has the advantages of an in-line location to minimize measurement delay and a "partial Couette" flow geometry, which affords independent control of the shear rate. The reactive extrusion process for ethylene methacrylic acid (EMAA) ionomer neutralization was modeled as a first order plus dead time process. A comparison of the process responses to experimentally determined residence time distributions for the extruder showed that there was an appreciable inherent delay despite the advantage of the rheometer’s in-line location. Proportional-integral and minimum variance control algorithms were implemented. The quality of control was generally good, but demonstrated the nonlinearity of the neutralization process, suggesting that adaptive control techniques could improve control performance.