The γ-ray polymerization of tricosa-10,12-diynoic acid (CH3-(CH2)9-C≡C-C≡C-(CH2)8-COOH) crystals has been studied by X-ray diffraction, optical- and scanning electron-microscopy and Raman- and optical-spectroscopy. Up to a conversion of about 60%, the bulk of the polymerization proceeds in a homogeneous single phase. Above about 50% conversion a phase transition, nucleating at crystal defects, occurs. Defect free crystals, which remain in the initial phase above 60% conversion, can be transformed by mechanical deformation. The transformation to the new phase destroys the macroscopic crystalline order but allows further polymerization at a reduced rate.