Stereospecificity of the polymerization of tert-butylacetylene by Mo-based catalysts was investigated. The polymer obtained by the MoOCl4-n . Bu(4)Sn system just after complete monomer consumption had an all-cis structure. However, the stereoregularity decreased when the polymerization was allowed to stand further. These findings indicate that, though the propagation reaction proceeds stereospecifically, the geometric isomerization of polymer lowers stereoregularity. This isomerization was catalyzed by several acids as well as by polymerization catalysts and, hence, is an acid-catalyzed reaction. Highly stereoregular polymer could be obtained by accelerating polymerization (by use of cocatalysts) and/or by decelerating isomerization (by use of cocatalysts, poor solvents of the polymer, or the less acidic Mo(CO)(6)-based catalyst and by polymerization at low temperatures).