Commercial ethylene-octene linear low-density polyethylene (LLDPE) polymers were reactively extruded with low levels of 2,5-dimethyl-2,5 di(t-butylperoxy) hexane to modify their molecular structure and processing properties. Peroxide levels were kept low to avoid crosslinking. This article examines the effects of reactive extrusion in a corotating intermeshing extruder. Gel content analyses and examination of extruded thin tapes indicated that the products were gel-free, but line-broadening in high-resolution C-13-NMR spectra suggested that some crosslinking did occur. Molecular weight distributions were broadened toward higher molecular weights, as expected. SEC estimates of long-chain branching in reacted polyethylenes were consistent with the results of C-13-NMR analyses. Under our extrusion conditions, the products contained about one long branch per number-average molecule. This result and data on changes in carbon-carbon unsaturation indicate that the major chain extension mechanism is an end-linking reaction between terminal vinyls or allylic radicals formed at chain ends and secondary radicals. Both types are produced by hydrogen abstraction on the LLDPE. All long branches originated at tertiary branch points. Changes in thermal behavior, as measured by DSC analyses, paralleled those observed by temperature-rising elution fractionation (TREF). SEC molecular weight measurements and long-branch determinations by SEC and C-13-NMR can be used to quantify the effects of peroxide treatment on the molecular structure of polyethylenes. DSC and TREF techniques, however, appear to be more sensitive than are SEC or NMR. Relatively minor variations in the degree of mixing and temperature control during reactive extrusion have noticeable effects on the molecular structures of the peroxide-treated LLDPEs.