Phase separation trends in blends of poly(vinyl methyl ether) (PVME) with various hydrogenous block-deuterated polystyrenes (PS) have been studied by a fluorescence technique. In the case of linear polystyrenes of a given overall chain length and deuterium content, the position of the LCST-type phase diagrams in the temperature-composition plane is shown to depend on whether the deuterated block is located in the middle or at the end of the chain. Additionally, it is observed that the kinetics of spinodal decomposition differ from one system to the other. The inspection of the phase behavior of mixtures, including star-shaped hydrogenous-block-deuterated PS’s, and the investigation of the solubility properties of the PS copolymers in a monomeric analog of PVME support the idea that the polymer-polymer interaction parameter depends on copolymer morphology, namely, on the accessibility of the ether functions to the thermodynamically-favored deuterated phenyl rings of polystyrene.