Segmental dynamics of two phase-separated reactively prepared polystyrene blends namely unsaturated polyester resin (UPR) and high impact polystyrene (HIPS) was investigated by dynamic-mechanical spectroscopy and calorimetric studies. The results showed a thermorheologically simple behavior for the HIPS, which could be quantified based on the KWW function with beta(KWW) of 0.37. The UPR data, however, could not be evaluated using KWW function, even though lower beta(KWW) than the HIPS was expected for it. Furthermore, the cc-dispersion of the UPR was considerably broader while its fragility index was comparable with the HIPS. Nonetheless, segmental dynamics comparison based on normalized Delta C-p(T-g) by molecular weight of the structural units of the studied systems showed much greater differences. Accordingly, the UPR was both kinetically and thermodynamically more fragile than the HIPS. The higher fragility of the UPR could be attributed to its larger relative cooperativity size and topological constraints. Finally, enhanced contrast in dynamic fragilities of two studied systems could be achieved if similar overall and local compositions could be made experimentally. (C) 2007 Wiley Periodicals, Inc.