The fine structure of acrylic fiber has been characterized using techniques based on gas adsorption, density, electron microscopy, and light microscoly. The techniques are complementary and have heen applied to acrylic fiber prepared under current commercial conditions. The existence of a fibril-void type of structure, with voids and fibrils of the order of 200 Å diameter, has been established for wet-spun fibers. This structure was shown to originate during coagulation and has been followed through stretching and collapsing. Dry-spun samples developed a similar structure, which becomes evident after the orientation step.