On the basis of phase separation temperatures (T-s) for poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends of different compositions, measured by small angle laser light scattering, we have studied dynamic theological behavior of these blends at different temperatures below, near and above T-s. The results showed that the time-temperature superposition principle could be applied and the linear viscoelastic response emerged in the terminal region at temperatures below T-g. However, it was found that the breakdown of time-temperature superposition principle appeared, and viscoelastic response deviated from the linear behavior in the terminal region at temperatures near and above T-g.