The production of carbon fiber from bio-based or renewable resources has gained considerable attention in recent years with much of the focus upon cellulose, lignin, and cellulose-lignin composite precursor fibers. A critical step in optimizing the manufacture of carbon fiber is the stabilization process, through which the chemical and physical structure of the precursor fiber is transformed, allowing it to withstand very high temperatures. In this work, thermogravimetric analysis (TGA) is used to explore and optimize stabilization by simulating different stabilization profiles. Using this approach, we explore the influence of atmosphere (nitrogen or air), cellulose-lignin composition, and alternative catalysts on the carbon yield, efficiency, and rate of stabilization. Carbon dioxide and water vapor released during stabilization are analyzed by Fourier transform infrared (FTIR) spectroscopy, providing further information about the stabilization mechanism and the accelerating effect of oxygen and increased char yield (carbon content), especially for lignin. A range of different catalysts are evaluated for their ability to enhance the char yield, and a phosphorus-based flame retardant (H3PO4) proved to be the most effective; in fact, a doubling of the char yield was observed.