The stabilization stage in the processing of carbon–silicon alloy (CSA) precursor fibers is investigated in this study. The critical stabilization parameters are identified and shown to control the mechanical properties of fibers both at the stabilization stage and, after further pyrolysis and controlled oxidation, to produce oxidation-resistant fibers. The attainment of infusibility in the stabilized fibers, necessary for the production of CSA fibers, determines the lowest stabilization degree, whereas the highest stabilization degree can be identified from the relationship between stabilization temperature and tensile properties of CSA fibers, thus enabling the optimum stabilization conditions to be determined. The CSA fibers produced by proper control of stabilization conditions significantly enhance mechanical properties, which are more than double those of CSA fibers obtained previously. Fourier transform infrared spectroscopy and nuclear magnetic resonance studies show that at stabilization temperatures above the optimum there is significant formation of silica in the stabilized fibers. This leads to a higher modulus but lower tensile strength and elongation.