Solid state polymerization has the potential to afford topochemical control over reaction pathways. High-pressure solid state polymerization at low temperature may permit a single lowest energy reaction pathway to be favored and also allow reactants to approach more closely before reaction begins, both factors that may favor topochemical reaction. Acetylene begins to polymerize in the solid state at a pressure of 12.5 cpa at 77 K. Further reaction occurs upon release of pressure to 2.5 GPa, suggesting the presence of unsatisfied reactive sites at high pressure. A topochemical polymerization did not result, but Raman spectroscopy indicates the formation of some polymer, which exhibits a spectrum characteristic of cis-polyacetylene. This polymer isomerized to the trans form upon warming to 298 K.