Polyacetylenes with substituents were, in general, more thermally stable than polyacetylene in air. Their stability increased with increasing number and/or bulkiness of the substituents: -[CH=C(n-alkyl)]_n-< -[CH=CPh]_n-, -[CMe=C(n-alkyl)]_n- < -[CH=C(t-Bu)]_n-, -[CMe=C(SiMe₃)]_n- < -[C(n-al-kyl)=CPh]_n-, -[CCl=CPh]_n- Among them, the polymers of aromatic disubstituted acetylenes were extremely stable (e.g., no oxidation or degradation in air at 160 °C after 20 h). The polymers from aliphatic dsubstituted acetylenes were moderately stable at room temperature, while they easily degraded at higher temperature. None of these polymers underwent degradation in vacuo at 120 °C, which indicates that the oxidation process is essential to the thermal degradation. Products of polymer degradation in air contained appreciable amounts of oxygen. According to the thermogravimetric analysis of the polymers in air, the starting temperature of weight loss was as follows: -[CH=C(alkyl)_n. 150 °C; -[CMe=C(n-alkyl)]_n-, ca. 200 °C; -[CMe=CPh]_n-and -[CCl=CPh]_n-, ca. 300 °C.