AbstractDilatometric measurements have been carried out on the crystallization of poly‐acrylonitrile from propylene carbonate solutions at concentrations of 1.0, 4.2, and 8.4% and temperatures of 40, 70, 95, 104, and 108°C. Contrary to expectation, the volume of the solution actually increases upon crystallization, although the increase is extremely small (about 1% based on polymer). This small change in volume is consistent with the partial specific volume of polyacrylonitrile in solution and the specific volume of the polymer measured directly by a flotation method. Recent investigation by Holland, et al. revealed that polyacrylonitrile, upon crystallization from dilute solution, forms crystalline platelets resembling the single crystals observed in other polymers. It appears that the small change in volume on transformation from the dissolved state into the crystalline state is due to the closeness of the specific volumes in the two different states. These unusual density data of polyacrylonitrile resemble those of poly(methyl‐methacrylate), where isotactic, syndiotactic, and atactic polymers have about the same density. The fact that the difference in density of polyacrylonitrile is so small might well explain a number of related phenomena: the lack of abrupt volume change on melting, the absence of amorphous regions, the narrow range of the density values of different samples prepared by different methods, and the fact that crystallization conditions exert practically no effect on the density of the sample.