Polyolefines are frequently functionalized with polar monomers via peroxide-initiated grafting that starts at macroradicals. However, polyolefine macroradicals also undergo undesired secondary reactions. Polypropylene (PP) is degraded, while polyethylene (PE) is crosslinked. Mechanistically, PP radicals are split by beta scission while PE radicals recombine to clusters. If these opposed tendencies can be balanced in PP-PE blends was investigated in this study. In principle, coupling of PP and PE radicals can lead to graft copolymers PE-g-PP. But the chances for graft reactions are good only in homogenous PP-PE blend melts. It is difficult to decide whether these blends are, in fact, in the melt one-phase or two-phase systems because molten PP and PE are too similar in all respects. PP-PE blends were processed with peroxide in the melt and, for comparison, also in solution. According to viscosity and gel permeation chromatography and differential scanning calorimetry results, graft reactions occurred only in the (presumably homogeneous) blend solutions in which degradation was subdued and crosslinking prevented. In PP-PE blend melts, on the contrary, the two polymers reacted fairly independently of each other. PP was degraded, and PE crosslinked. Apparently, these blend melts, although transparent, are two-phase systems.