Substituted poly(paraphenylene! derivatives (PX) have recently gained interest due to their unique properties [1-5]. They are amorphous thermoplastics with relatively modest glass transition temperatures (T-g similar to 150 degreesC), are soluble in various common organic solvents, and yet possess the strongest mechanical properties of any amorphous thermoplastic [6]. However, the blend behavior of these polymers is relatively unexplored. Here we report the phase behavior of PX and various thermoplastic blends and highlight the PX and polycarbonate (PC) blends which were found to possess a lower critical solution temperature (LCST) as determined through enthalpy relaxation and cloud point measurements. The morphology and storage moduli of the PX/PC blends have been observed in both the single phase and two phase regimes.