The mechanical properties of aromatic and heteroatom-containing aliphatic polyacetylenes were examined. Regarding tensile properties, aromatic polyacetylenes [i.e. ring-substituted poly(phenylacetylene)s and poly(diphenylacetylene)s] showed large Young’s moduli (500-2000 MPa) and small elongations at break (1-8%), implying that these polymers are hard and brittle. In contrast, Young’s moduli of Si- and S-containing aliphatic polyacetylenes were rather small (several hundred megapascal), and their elongations at break were relatively large (several tens per cent); i.e. they are soft and ductile. This tendency was remarkable in the polymers having long n-alkyl chains. According to dynamic viscoelastic measurements, the glass transition temperatures (T(g)s) of poly(diphenylacetylene)s were > 200-degrees-C, whereas those of poly(phenylacetylene)s were 110-200-degrees-C, depending on the kinds of substitutents. The T(g)s of aliphatic polyacetylenes are 40-160-degrees-C, lower than those of aromatic polyacetylenes. The effects of substituents on T(g) are discussed in comparison with those of vinyl polymers.