A theological study of the phase behaviour and the the immiscible region of a model partially miscible Upper Critical Solution Temperature (UCST) blend was carried out by using a series of nearly monodisperse polydimethylsiloxane (PDMS) of varying molecular weights blended with polyethyl-siloxane (PEMS). The complete phase diagram was established through steady shear theology, optical microscopy and light scattering. The UCST, interfacial tension morphology of these blends are very sensitive to the molecular weight, of the constituents. Partial miscibility was observed only when the M-W of PDMS was sufficiently low. The viscoelastic behaviour in the phase separated region can be modeled successfully,using the Palierne model.