A new class of compatibilizers suitable for blends or alloys of polypropylene and engineering polymers having aromatic residues or functionality complimentary to hydroxyl were evaluated in blends of isotatctic polypropylene (PP) and polyamide 6 (PA6). The compatibilizer consisted of a PP part with a phenol formaldehyde (PF) polymer grafted onto it, in this study, various combinations of the polymer parameter of each compatibilizer building block were examined. Based on the same loading, the compatibilizer with low molecular weight PP and high content of high molecular weight PF was observed to be the most efficient. A compatibilizer content of up to 7.5% by weight gave significant reduction in the average particle size of the dispersed PA phase. Similarly, corresponding improvements in the mechanical properties were observed as the average particle size was reduced. For some of the blends, more than additive improvement in the mechanical properties were achieved. (C) 2000 John Wiley & Sons, Inc.