Commercial ethylene-octene linear low-density polyethylenes (LLDPEs) were reactively extruded with low levels of a peroxide [2,5-dimethyl-2,5-di(t-butylperoxy) hexane] to modify polymer molecular structure and processing properties. Peroxide levels were kept low to avoid crosslinking. This article reports the effects of this reactive extrusion on viscoelastic properties. Rheological properties are more sensitive than are molecular structure characteristics to the changes produced by reactions of very low peroxide concentrations. Complex viscosity increases are seen, especially at low frequencies. Shear-thinning behavior is also accentuated. The crossover between G’ and G " moves to lower frequencies. A modified Cole-Cole presentation of these data shows that the elastic component is more predominant for extrusion-reacted materials. Peroxide-modified materials all have higher Bagley end correction values than those of barefoot resins. The former exhibit lower power indices (more shear thinning). All these properties indicate more long-chain branching and higher melt elasticity. However, die swell decreased as a function of peroxide concentration. Peroxide treatment results in an enhancement of elongational viscosity, both under isothermal and cooling conditions, along with a decrease in drawdown ability. The theological changes parallel those reported earlier in molecular characteristics but are more sensitive and suitable to evaluate the effects of reactive processing. The effectiveness of the reactive extrusion process for improving processability of LLDPEs depends critically on the extrusion conditions.