The purpose of this study is to determine to what extent CO2 can be used as a processing aid to melt process polyethylenes of higher molecular weight than can be typically melt processed. To assess the ability to melt process high molecular weight polyethylenes with CO2, the viscosity of a 460000 g/mol high density polyethylene (HDPE) plasticized with various amounts of absorbed CO2 in an extrusion process was determined using a slit die rheometer that was attached to the end of the extruder. A relationship was developed to determine the maximum molecular weight polyethylene that could be processed at a given viscosity reduction due to absorbed CO2. This relationship was tested by using CO2 to process a blend of 40 weight percent ultra-high molecular weight polyethylene (UHMWPE) and the 460 000 g/mol HDPE which lead to an equivalent molecular weight of 720000 g/mol. The viscosity of the blend with 12 weight percent CO2 was reduced to that of the pure 460 000 g/mol HDPE as predicted by the relationship. Preliminary studies using a pressurized chamber attached to the exit of the die allowed one to assess the conditions under which suppression of foaming within fibers or other extruded forms is possible.