A mathematical model for the changes in molecular weight distribution that occur when free-radical initiator is added to a blend of linear low-density polyethylene and polypropylene during extrusion was developed. Experimental verification employed a size-exclusion chromatograph equipped with both differential refractive index detection and a solvent-evaporation interface to enable infrared detection. The model combines two previously published models (a polypropylene degradation model and a polyethylene modification model) using a single parameter : an initiator partition coefficient. All of these models were shown to provide excellent fits to new reactive extrusion data. However, with one of the initiators used, the polyethylene modification model required values of initiator efficiency greater than unity. This result was also recently observed in another study and may be due to localized variations in peroxide concentration or to an inadequacy in the reactions specified for polyethylene with this initiator.