The rheological properties of high-density polyethylene melts were found to change drastically after treatment with oxygen or peroxide. Unusual features of the treated melts in shear flow (190degreesC) included a) increase in length of time to reach steady state values of shear stress in start-up experiments; b) a non-reproducibility of the low-shear rate sections of the flow curves measured at increasing and decreasing shear rate; c) an increase of viscosity at low shear nates compared to the neat sample. Under non-stationary extensional flow (a regime of constant force) the treatment leads to a change in shape of the strain development with time, an increase of the apparent elongational viscosity, and an increase in time to break. At 150-170degreesC, the rheological behavior of the treated polyethylenes is completely identical to the corresponding behavior of the untreated. These results, together with data from IR-spectroscopy and GPC suggest the following mechanism: The oxidation or peroxidation leads to reactive sites in the polymer chain that incorporate a few long branches during the initial contact with oxygen or peroxide. These reactive sites remain in the polymer after cooling/solidification and can become activated again upon heating to 190degreesC causing additional changes in molecular structure. Formation of the long-chain branches results in an increased resistance of the melt to extensional deformation, and an improvement in processing behavior, as well as the quality of bottles produced by the blow-molding process. (C) 2004 Society of Plastics Engineers.