In attempting to melt blend polymethylphenylsilane (PMPS) and polystyrene, an unusual morphology was observed which led to a more detailed examination of the thermal and physical properties of PMPS. Thermogravimetric analysis shows that PMPS is stable under the conditions used for the melt mixing. For apparently the first time, the results of differential scanning calorimetry, dynamic mechanical thermal analysis and thermomechanical analysis art reported for PMPS. The curves obtained by these three techniques exhibit two thermal transitions at temperatures which are strongly molecular weight dependent. Optical microscopy and X-ray diffraction results show some evidence of the existence of long range ordered regions in PMPS. From the combination of thermal, microscopic and X-ray diffraction results, we assigned the first thermal transition to a glass transition and the second one, occurring at high temperature, to a phase-disordering transition accompanied by the softening of the polymer.