On the basis of an online sampling microscopy method, the morphological evolution of a metallocene polyethylene/metallocene ethylene-propylene copolymer system (80/20 vol %) across various mixing regimes was investigated and treated statistically. The size distributions of the minor-phase metallocene ethylene-propylene (mEP) droplets were described with principles of irreversible thermodynamics. Such an approach allowed us to find two superimposed statistical ensembles involving primary (broken) and secondary (coalesced) mEP particles. The mean size and relative number of both broken and coalesced mEP particles were calculated. The evolution of these characteristics across melt mixing, static coalescence, and flow-driven coalescence was analyzed. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3421-3431, 2013