This article reports the results of an investigation into the time-dependent morphological and rheological changes that accompany the in-situ polymerization of blends composed of poly(hydroxyether of bisphenol A) (phenoxy) and poly(styrene-co-acrylonitrile) (SAN). The rheological behavior was monitored continuously during the in-situ polymerization, whereas the miscibility and phase structure of blends formed in situ were examined at discrete stages of polymerization by differential scanning calorimetry and transmission electron microscopy. In the blend with 30 wt % SAN, a co-continuous blend morphology was associated with gradual changes in the dynamic moduli, suggesting that phase separation proceeded by spinodal decomposition (SD). In contrast, phenoxy-rich dispersions were uniformly dispersed in a continuous SAN-rich matrix in the blend with 50 wt % SAN, and the corresponding rheological signature revealed a sharp initial increase in the dynamic moduli, followed by slower growth after long times, indicative of phase separation via nucleation and growth (NG). The rheological property changes are closely related to morphology development and mechanisms of phase separation induced during in-situ polymerization. (c) 2007 Wiley Periodicals, Inc.