The formation of microvoids in polyacrylonitrile (PAN) fibers was studied as a function of temperature by synchrotron small-angle X-ray scattering (SAXS). The microvoids show preferred orientation and have elongated shapes with lengths in the range of 30 to 100 nm and diameters in the range of 0.5 to 3.0 nm. In the pristine fibers, the microvoid content is of the order of one volume percent, the length is 30-40 nm, and the diameter 2-3 nm. It was found that varying numbers of additional microvoids are formed in a temperature range of 190 degrees C to 470 degrees C. In the final stage, the content of microvoids is of the order of 10-20 volume percent, and their lengths and diameters are 40 and 0.5 nm, respectively. The formation of the microvoids is correlated with the cyclization and aromatization reactions of the fibers, which are important steps in the formation of the carbon fiber structure. A detailed SAXS evaluation procedure was used for the quantitative description of orientation, length, diameter, and relative volume of microvoids. The procedure may be of interest for the characterization of microvoid systems in other types of fibers.