Renewable resources, such as kraft lignin, have shown great potential as precursors for carbon fiber production. This manuscript reports an investigation into the stabilization of softwood kraft lignin (SKL) fibers and the determination of the difference in stabilization between hardwood- and softwood-based kraft lignin fibers. The stabilization was achieved either thermally by using only heat or oxidatively in the presence of air, at various heating rates. A heating rate of 4 degrees C min1 and a holding time of 30 min at 250 degrees C were successfully used for the thermal stabilization experiments. Faster stabilization was achieved using oxidative conditions at a heating rate of 15 degrees C min1 and 30 min holding time at 250 degrees C. Furthermore, stabilization and carbonization in a one-step process was performed on SKL fibers, which show great potential to reduce both production time and costs. The stabilized and carbonized fibers were evaluated using thermal, spectroscopic, and microscopic methods. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013