Mulberry and tasar silk fibers were treated with formic acid, lithium thiocyanate, and zinc chloride. The fine structure of these preswollen fibers was investigated using infrared spectroscopy, x-ray diffraction, and sonic velocity. Densities were determined by means of a liquid density gradient column, and the sonic velocity was measured using a dynamic modulus tester. The surface morphology of swollen fibers was studied using electron microscopy. The x-ray and infrared crystallinity indices of fibers change due to swelling treatments, and these changes correlate to the changes in density and sonic velocity. -- AATA