It was found that poly-β-vinylnaphthalene and poly(ethylene oxide) (54:46% by weight) form homogeneous blends or graft copolymers when prepared under conditions which inhibit the crystallization of poly(ethylene oxide). Such preparations can be obtained from solutions in a good solvent for both components, either by precipitation procedures which lead to coacervation rather than fractionation, or by freeze-drying. Preparations containing more than 46 wt.-% poly(ethylene oxide) were characterized by modulus versus temperature curves exhibiting a minimum at about 60°C. This behavior was interpreted as due to microphase separation taking place with increasing temperature, a process which becomes irreversible above 120°C. A graft copolymer having the 54:46 composition was rubbery at room temperature, and in contrast to polyblends did not exhibit the poly-β-vinylnaphthalene glass transition at 138°C. The marked difference in behavior between polyblends and graft copolymers is due to the fact that in the latter irreversible microphase separation cannot take place. The above described properties seem to be peculiar to poly-β-vinylnaphthalene. Similar properties could not be reproduced in preparations of poly(ethylene oxide) with polystyrene, poly-4-vinylbiphenyl, or polyacenaphthylene. These preparations exhibited a behavior characteristic of mixtures of two incompatible polymers.