The severe thermodynamic conditions of the high-pressure ethylene polymerization process hinder ethylene from going to full conversion. One remedy to improve the monomer conversion is to make effective use of difunctional peroxides. Multifunctional peroxides can accelerate the polymerization rate, produce branching, and modify the rheological properties of molten polymers. This article proposes a kinetic model based on a postulated reaction mechanism for ethylene polymerization initiated by difunctional initiators in a high-pressure tubular reactor. Three peroxides suitable for ethylene polymerization were compared for their effectiveness. Compared to dioctanoyl peroxide, the two difunctional peroxides considered performed much better for the higher temperature regions of the reactor and gave ethylene conversions nearly twice as high for only half of the initial amount of dioctanoyl. They also generated low-density polyethylene polymer with a broader molecular weight distribution and longer chain branching. These two important polymer characteristics can influence the end-product rheological properties. Injecting fresh ethylene at different points along the reactor improved the conversion and produced more branched polymer. (C) 2008 Wiley Periodicals, Inc.