Acetylene-containing polymers, obtained by the radical polymerization of 4-substituted aromatic enynes, were subjected to the thermal crosslinking reaction on the basis of the reactivity of the acetylene moieties. The crosslinking occurred from ca. 140 degreesC to give an insoluble product. The crosslinking was supposed to be the irreversible reaction of the acetylene moieties to give new conjugated structure from the IR and UV-vis spectra. The steric bulkiness of the substituents tend to restrain the crosslinking and decline the decomposition temperature of the polymers.