The temperature dependences of the absolute small angle X-ray scattering powers and of the unit cell spacings of a series of 1,3,5-trioxane—1,3-dioxolane copolymers were measured. From a comparison of these data and of the specific volume values measured by Wilski it is concluded that values of crystallinity calculated in a conventional manner provide meaningful parameters when discussing the microstructure of these materials. The measurements were made on annealed samples. At temperatures below the original annealing temperatures changes in small angle scattering power, long periods and crystallinities of the samples were observed. These changes were reversible with temperature and were to…