To explore the possibility of producing branched polypropylene (PP) by a reactive extrusion (REX) process, isotactic PP was reacted with a polyfunctional monomer, pentaerythritol triacrylate (PETA), in the presence of 2,5-dimethyl-2,5(t-butylperoxy) hexane peroxide (Lupersol 101). Experiments were carried out in an intermeshing, corotating twin-screw extruder at 200Β°C using three concentrations of peroxide (200, 600, and 1000 ppm) and four concentrations of PETA (0.64, 1.8, 2.8, and 5.0%, by weight). Shear viscosity and MFI of the whole polymers was found to increase with PETA concentration and decrease with increasing the peroxide concentration at a given PETA concentration. The macrogel amount in the materials produced was determined in refluxing xylene using Soxhlet extraction and at PETA concentrations higher than 1.8 wt % the macrogel content increased with increasing peroxide concentration. No macrogel was detected at low PETA concentrations ( ~0 . 6 4 % ) a t all three peroxide levels, suggesting that low concentrations of PETA and peroxide should be used in order to minimize the formation of macrogels. The xylene soluble portions (sols) of the modified materials were characterized by FTIR and DSC. Generally, the relative intensities A1740/A841 in the FTIR spectra increased with increasing PETA incorporated into PP. Two melting peaks ( T m l and Tm2) were observed in the DSC traces of some of the sols, and the crystallization temperatures (T,) were higher than those of the virgin and degraded polypropylenes. The DSC behavior of the sols suggests that the modified PPs contain branched and/or lightly crosslinked chain structures.