After cis-poly(phenylacetylene)s (cis-PPAs) were heated at 180 degrees C for 1 h, the M(n)s decreased from 5000-53 000 to 2000-2600, 1,3,5-TPB corresponding to the cyclic trimer of phenylacetylene was formed, and the yields of the annealed PPA and 1,3,5-triphenylbenzene (1,3,5-TPB) did not significantly vary. A peak at 280 nm due to the phenyl ring and broad band up to 600 nn due to the pi-pi* transition of the main-chain were observed in the u.v./vis. spectrum of cis-PPA. On the other hand, the absorption due to the conjugated main-chain disappeared for annealed PPA, though the phenyl group absorption was detected. The disappearance of the main-chain absorption can be explained by deflection of the main-chain.