AbstractDomains, or areas of inhomogeneities and imperfections, may be made visible in polystyrene by treatments of the plastic with certain liquids and vapors. Different types of domains are revealed by different techniques, depending upon the solvent power of the liquids and vapors. One type of domain reveals itself as craze cracks when polystyrene is first soaked in a nonsolvent such as methanol, and then exposed to the vapors of a solvent such as hexane or benzene. A second type of domain may be revealed by exposing polystyrene to certain mixtures of solvents and nonsolvents which produce a rough, whitened surface on the polystyrene. For the first type of crazing, the size of the domains is influenced by the frozen‐in thermal stresses, and the shape of the domains is influenced by molecular orientation. The domains of the second type are due to heterogeneity of molecular orientation and are not found in annealed specimens. A theory has been proposed which explains crack crazing in terms of adsorbed vapors in cracks and areas of imperfections in the polystyren