The early stage of the morphology development has been studied for the blending of two immiscible polymers. Controlled experiments were carried out in a batch mixer in such a way that the rate of melting was low enough to follow up the morphology development of dilute and concentrated systems. For a dilute or semidilute polypropylene and polyamide 6 (wPP/PA6) blend with 0.5, 5, or 10 wt % PA6, particles formed in the very early stage of melt blending were very small, of the order of 0.25 to 0.3 mum in radius. They immediately began to grow in size when no compatibilizer was added, indicative of coalescence even in the very early stage of melt blending and/or in very dilute systems (0.5 wt % PA6). Further growth of the particles was eliminated with the introduction of a graft copolymer compatibilizer providing evidence of the stabilizing effect of the copolymer from the very beginning of melting blending. However, the behavior of the morphology development of a concentrated PP/PA6 (80/20) system was similar to that reported in the literature. The average radius of the particles of the uncompatibilized blend decreased with increasing mixing time, whereas that of the compatibilized blend remained almost constant during mixing. The most favorable conditions to obtain a fine morphology seems to be the following: rate of melting/plastification of pellets < rate of dispersion (deformation + breakup) of the polymer melt to small particles < rate of stabilization (with an adequate copolymer). (C) 2001 John Wiley & Sons, Inc.