A software program for experimental design and response surface methodology (RSM) was used to set up and evaluate the effect of modifiers in mixed polyolefin fractions. Sheet extruded blends of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), 2 different high-density polyethylenes (HDPE) and 2 different polypropylenes (PP) were prepared and characterized by tensile tests and tear tests. Ethylene-vinyl acetate copolymer (EVA), a very-low-density polyethylene (VLDPE) and a poly(1-butene) (PB) were used as modifiers at levels up to 20 wt % in order to improve the toughness of PE-PP blends. LLDPE, in which 1-butene is the comonomer, is most compatible with the PPs. The VLDPE generally gives the most substantial gain in tensile strength and tear propagation resistance, and it also has the ability to smooth out effects of variations in the composition of PE-PP mixtures. For HDPE-PP blends, the EVA and PB are also effective modifiers. PB acts as a plasticizer in the PP phase whereas VLDPE forms a separate rubbery phase. The RSM is a valuable tool in, for example, recycling operations, where mixed plastic fractions often appear.