We prepared partly crystalline polydiacetylenes (PDAs) by the solid-state polymerization of substituted benzyl 10,12-alkyldiynoates followed by the dissolution of the resulting polymers in refluxing 1,2-dichlorobenzene and subsequent reprecipitation. The obtained PDAs were soluble in organic solvents, such as chloroform and tetrahydrofuran, at room temperature. They showed thermochromism not only in the bulk and polymer matrices but also in solution. The UV-vis absorption spectrum of PDAs in Solutions changed depending on the temperature along with a large hysteresis. The thermochromic properties were dependent on the structure of the PDAs, that is, the length of the alkyl side chains and also the structure of the substituent on the phenyl group. We have found that aggregates are formed in the solution when the temperature decreased on the basis of the NMR and dynamic light scattering measurement results.