Diffuse small-angle X-ray scattering is caused by inhomogeneity in the electron density distribution of a material and is often attributed to the existence of microvoids in polymers. The content, dimensions and relative surfaces of the microvoid system can be determined thereby. The dimensions of the coherently scattering domains and the sizes of the inhomogeneities are related by the form, or shape factor (f). Changes in this factor highlight structural changes in polymer fibers caused by different treatments. In the present paper the microvoid system of differently treated poly(ethylene terephthalate) (PET) and poly (propylene) (PP) fibers were studied after subjugation to different treatments. PP fibers were spun from two different polymers and drawn at different temperatures, whereas PET fibers were treated in different media. The form factors found for PET and PP fibers were rather high. For PP fibers, drawn at different temperatures and draw ratios, the form factors were between 2 and 5. Untreated PET samples had form factors of about 6. A supercritical fluid treatment could increase the form factor up to 8. Conventional water dyeing and hot air treatments caused considerable changes in the microvoid system and increased the form factor to values even larger than 8.