cis-Poly(phenylacetylene) (PPA) and cis-poly(pentadeuteriophenylacetylene) (PPA-d(5)) undergo similar thermally induced transformations in bulk and solution. The structures identified in thermally treated samples include cis-trans isomerization of the backbone, cyclohexadiene repeat units in the main chain, and extruded 1,3,5-triphenylbenzene (1,3,5-C6H3Ph3). Among these structures, cyclohexadiene sequences along the backbone formed by intramolecular cyclization are the principal products in all solvents regardless of temperature and the presence or absence of ambient light or O-2. We propose that cyclohexadiene structures form via 6,7 electrocyclization of triene sequences and, therefore, are an unavoidable consequence of cis-alkene units of the polyene backbone. Concurrent with cyclization, decrease of molecular weight and increase of molecular weight distribution are observed. Traces of acid lead to a dramatic increase in cyclization. Extrusion of 1,3,5-C6H3Ph3 results from cyclohexadiene repeat units and is slower than electrocyclization, even in the presence of acid, light, or O-2. In the presence of ambient light, O-2 accelerates the rate of cyclization and extrusion of 1,3,5-C6H3Ph3, the decrease of molecular weight, and the formation of carbonyl-containing products detected in a 5:1 molar ratio to extruded 1,3,5-C6H3Ph3. Under these conditions, the most dramatic changes in molecular weight and molecular weight distribution are observed.