Mechanical and rheological properties of blends of polypropylene (PP) and linear polyethylene (PE) were studied, with particular attention to the effects of aging of such mixtures. These two olefin polymers are basically incompatible. In PP-rich blends, addition of high-density PE (HDPE) causes only a slight decrease in tensile properties and impact resistance of injection-molded specimens. In all cases, annealed specimens have higher moduli and lower impact strength than as-molded products. While none of these changes are very drastic, the addition of small amounts of HDPE was observed to result in a serious decrease of gate-region impact resistance of thin-walled moldings. Blends with 10-20% HDPE exhibited an unexpected interaction in tensile, thermal, and melt-flow properties as well as in crystallization behavior.