The stability of poly(phenylacetylene)s in solution was systematically studied. Cis-transoidal poly(phenylacetylene) prepared with a Rh catalyst readily underwent autoxidative degradation into oligomers, whereas a W-based polymer with a trans-rich geometrical structure degraded slowly under similar conditions. The degradation of W-based poly(phenylacetylene) was independent of the solvents, whereas the degradation of the cis-transoidal material strongly depended on the solvents, and very rapid degradation was observed in toluene and CHCl3. A reduction in the degradation rate in the absence of oxygen and the appearance of carbonyl groups in an IR spectrum and aldehyde protons in a H-1 NMR spectrum of the resulting oligomers indicated the direct participation of oxygen in the degradation where light was supposed to assist the degradation by producing radicals on the main chain. The cis content of cis-transoidal poly(phenylacetylene) was unchanged during the degradation, unlike for the acid-catalyzed decomposition, in which the cis content decreased linearly as the molecular weight decreased. Ring-substituted poly(phenylacetylene)s degraded slowly compared with poly(phenylacetylene), regardless of the kind of substituent, which suggests that steric factors affected the degradation rate. The use of radical scavengers such as 2,2,6,6-tetramethylpiperidine-1-oxyl and diphenylpicrylhydrazil suppressed the decomposition.