Abstract Tensile testing of polyoxymethylene fibers formed by solid-state polymerization of trioxane needles has been performed. Specimens were polymerized by an isochronal, isothermal postirradiation heat treatment following irradiation at constant dose rate but at varying total dose (1.55–7.44 Mrad). It is found that the elastic modulus, “yield strength,” and ultimate tensile strength increase smoothly with irradiation dose up to a dose of 3.88 Mrad at room temperature. There were, however, significant decreases in all mechanical properties measured in going from 3.88 to 7.44 Mrad (second highest to highest dose). X-ray rotation photographs indicate no observable difference in structure or orientation for the several specimens. Density gradient column results show a density peaking at 3.88 Mrad dose. A small peak in melting point vs irradiation time is also noted at this dose level. Finally and most markedly, the diameter of the fibrillar crystallites which make up the polymer fiber decreases with incre...