Process of detecting the presence of certain gases, including certain pollutants, in small concentrations; or to cause partial polymerization of certain polyacetylene compositions for use in time/temperature history indicators or radiation dosage indicators. The process involves contacting a reactive gas, such as a nitrogen oxide, a halide or ozone, with a crystalline substituted acetylenic monomer containing at least two conjugated triple bonds and substituents having at least one atom of oxygen or nitrogen. In particular the acetylenic compound is 2,4-hexadiyne-1,6-bis(phenylurethane) crystallized from tetrahydrofuran or acetone.