A blend of 10 vol % poly(methyl methacrylate) (PMMA) in a polystyrene (PS) matrix was prepared by mixing the polymers in the form of 1.0 mu m diameter latex particles, followed by melt pressing the powder at 110 degrees C into void-free films. These were annealed for various periods of time at 180 degrees C, and the aggregation and coalescence of the PMMA. microspheres were followed by laser confocal fluorescence microscopy. Contrast was provided by a fluorescent dye attached to the PMMA polymer. Brownian motion as a contributor to aggregation could be ruled out. The major factor leading to particle aggregation was identified as flow within the sample, likely due to inhomogeneities on heating, leading to convective fluxes in the sample. At early stages of annealing, Ostwald ripening contributed to the coarsening process.