Rheological data are reported for five fractions of poly(diethylsiloxane) (PDES) with molar masses of PDES ranging from 45 to 200 kg/mol. At room temperature, the samples exhibit a stable mesomorphic (biphasic) structure consisting of a mesophase (condis crystal) and an amorphous phase. The effects of molar mass and temperature on the theological properties of PDES are examined. At ambient temperatures, the high molar mass samples are highly elastic, as evident from the weak dependence of G' and G" on the frequency. This elastic behavior is attributed to the presence of mesophase domains that act as effective crosslinks in the melt. The mesophase melts to an amorphous phase on heating past an isotropization temperature that depends on molar mass. The biphasic and amorphous states exhibit large differences in their low frequency theological responses. The kinetics of mesophase formation is found to be dependent on the molar mass.