A novel bilayer-forming polyion complex, consisting of a cationic diacetylene Lipid (1) and a polyanion (poly(styrene sulfonate), PSS) has been assembled and immobilized on both a quartz plate and the core of a plastic optical fiber (POF), through which a He-Ne laser light passes continuously. The complexed cast film (1/PSS) shows a crystal-to-liquid crystal phase transition due to an ordered multibilayer structure. Upon UV irradiation, the film can be readily polymerized and gives the blue form of polydiacetylene. It is noteworthy that the polymerized film displays reversible thermochromism. Such polymerization and thermochromic behaviors, observed spectroscopically, are successfully monitored as He-Ne laser-signal changes by the use of a POF system. This system seems to have the ability to read out a more subtle molecular rearrangement in polydiacetylene assemblies as well as color changes due to formation of the blue polymer or thermochromism, since the laser light can draw information on refractive index of membrane phases. The pH responsiveness was also examined for a POF coated with the complexed film of a triblock polymer of PSS-block-poly(methacrylic acid)-block-PSS and 1.