Infrared spectra of poly[3,3-bis(chloromethyl)oxacyclobutane] (Penton) single crystals indicate that annealing perfects the crystals in a manner not involving a change in the relative amount of side-group rotational isomers. Other spectral and nonspectral considerations indicate that Penton has a highly crystalline but imperfect paracrystalline structure. The spectral differences between the two polymorphic forms are associated with differences in the magnitude of crystalline interactions rather than differences in relative amounts of side-group rotational isomers. An explanation for the nonthickening annealing behavior of Penton single crystals is offered.