An ethylene-octene linear low-density polyethylene (LLDPE) was treated with peroxide in a reactive extrusion system. A counterrotating nonintermeshing twin-screw extruder (System 2) was contrasted with a corotating intermeshing twin-screw machine (System 1). In System 2, the peroxide solution was pumped into the melted polymer, while it entered with the polymer pellets in the feed section of System 1. Molecular structure changes and the theological behavior of peroxide-modified resins are similar in both operations but System 2 is much more effective. Much lower peroxide levels were needed in System 2. However, reactions in this setup were also more difficult to control. The presence of microgel was clearly evident in System 2 products but not in those made in System 1. The results of such reactive extrusion processes depend critically on the method of the peroxide feed and mixing conditions. Reaction conditions that favor optimum economy and peroxide efficiency are those which may compromise product homogeneity.