Reactive blending of polysulfone (PSU) and polyamide 6 (PA) was carried out at 20:80 (PSU-PA) wt. ratio using a gram-scale mixer (Mini-Max Molder). Three PSUs with different functional groups were prepared and used: maleic anhydride-grafted PSU (PSU-MAH), carboxylic acid-grafted PSU (PSU-COOH) and phthalic anhydride-terminated PSU (PSU-PhAH), having almost same molecular weight (M-w = ca. 20 k) and functional group content (ca. 90 mu mole g(-1)). The change in PSU particle size with mixing time was investigated by light scattering and transmission electron microscopy. As expected, all reactive systems yielded finer particles via faster size reduction process, compared with non-reactive system. Attainable particle diameters were: ca. 1 mu m for non-functionalized PSU, ca. 0.6 mu m for PSU-MAH, 0.3 mu m for PSU-COOH, and ca. 40 nm for PSU-PhAH. The particle size of PSU-PhAH system was comparable to that in a pre-made block copolymer synthesized by solution polymerization, suggesting a potential for the solvent-free preparation of PSU-PA block copolymer when adequate reactivity and molecular architecture are provided in reactive blending.