A cis-poly(phenylene vinylene) (cis-PPV) recently synthesized by Katayama, Ozawa, and coworkers undergoes photo-isomerization into trans configuration. This isomerization occurred rather quickly within 45 min in solvent-cast films at room temperature. The cis-PPV chains exhibiting this quick isomerization were expected to be highly mobile in the bulk state, and this expectation was examined from thermal, structural, and viscoelastic tests for a cis-PPV sample with M-n = 4300 and M-w/M-n = 2.16. It turned out that the as-polymerized cis-PPV was partially crystalline and fully melted at T-m congruent to 109 degrees C. The samples quenched from a molten state to lower temperatures (<= 70 degrees C < T-m) as well as cast from a solution were amorphous for few hours, enabling the viscoelastic test in the fully amorphous state. The WLF-type time-temperature superposition was valid for these quenched and/or cast sample, confirming the amorphous character of the cis-PPV chains therein. The terminal viscoelastic relaxation time of these chains, equivalent to a time required for thermal changes in a large-scale conformation, was congruent to 10 s at room temperature. This result suggested that the conformational changes of the amorphous chains (the major component in the cast films) were sufficiently fast to allow the efficient cis-to-trans photo-isomerization.