The phase-separated structures formed in polymer mixtures consisting of a thermotropic liquid crystalline copolyester and poly(ethylene terephthalate) (PET) are investigated. The 50wt%/50wt% mixtures isotropic, transparent and homogeneous were prepared by dissolution of the copolyester and PET in an organic solvent followed by rapid removal of the solvent. Their phase separation processes during isothermal annealing at various temperatures after rapidly heating the as-prepared thin films were investigated by real-time and in situ polarized light microscopy and light scattering. At the annealing temperature below the melting point of PET (T-m,T-PET), the vitrification and crystallization of the components strongly affect phase separation, giving rise to pinning of the coarsening process at the different stages of phase separation and hence leading to various phase-separated structures. Rapid phase separation via spinodal decomposition was observed at annealing temperatures above T-m,T-PET. The phase-separated structures were found to evolve via the following sequence of mechanisms : (1) self-similar growth of the percolated networks of the anisotropic and isotropic phases; (2) disruption of the percolated network and formation of anisotropic droplets in the isotropic matrix; and (3) coalescence of anisotropic droplets. Light scattering from the percolated network structure was further investigated in terms of the spatial distributions of the orientation and concentration fluctuations.