Droplets of polymer blends flowing through convergent channels undergo collisions and coalescence because of the appropriate wineglass-shaped flow paths with essential flow constriction at the entrance zone. Therefore, an attempt has been undertaken to use capillary flow for studying coalescence phenomena in polymer blends. When the initial drop diameters in a barrel (before extrusion), d(br) and in the extrudate, d(er) are measured, coalescence efficiency can be easily calculated as E-c d(e)(3) /d(e')(3) provided that no breakup of elongated domains occurs. Compared with methods employing simple shear flow, it has several advantages. For example, the convergent flow pattern combining both shear and extensional flows is directly related to industrial processing operations like extrusion, injection molding, blowing, etc. The method imposes minor limitations on processing parameters and materials used. Applicability of the technique proposed was verified by systematic studies of coalescence in PMMA/PS binary melts blends during capillary extrusion and by comparing these results to theoretical predictions and experimental data from literature. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2724-2733, 2011