The mechanical and rheological properties of polypropylene (PP), polyamide 6 (PA6), and their blends treated by high-intensity ultrasound during extrusion were investigated. A lower head pressure was achieved in the extrusion of these thermoplastics. The mechanochemical and sonochemical effects of ultrasound led to simultaneous ionic condensation reactions and degradation in a homogeneous melt of PA6, with a prevailing effect of enhanced polycondensation reactions. The observed improvements in the mechanical properties of ultrasonically treated PA6 were attributed to condensation reactions, which yield a higher molecular weight, a higher crystallinity, and a more uniform crystal size distribution. At high ultrasound amplitudes, for PP, the degradation of polymer chains was observed with little deterioration of the mechanical properties. For ultrasonically treated PP/PA6 blends, a competition between the degradation and partial in situ compatibilization effect was found. At certain blend ratios, the tensile toughness and impact strength of the treated blends were almost double those of the untreated blends. However, full compatibilization was not achieved, possibly because of the low coupling selectivity of highly reactive radicals. (c) 2006 Wiley Periodicals, Inc.