The main subject of this work is an investigation of the effects of heating rate and current on the crystallisation of amorphous precursors in spark plasma sintering (SPS). For this, dry gel of Al₂O₃–SiO₂ with a molar ratio of 1:1, was synthesized and sintered in-situ by SPS, and also by hot pressing (HP) for comparison. Phase analysis showed that the only crystalline product in both cases was mullite, whose Al₂O₃ content was lower in the SPS specimens. The microstructures showed a low volume fraction of large mullite fibers in the SPS specimens, whereas a high volume fraction of fine equiaxed grains was present in the HP specimen. The main difference in microstructure between HP and SPS specimens could be explained in terms of the higher heating rate of the SPS specimens. The size of the SPS die also affected the size and aspect ratio of the mullite fibers produced, which might have been due to either the different electrical current required or a difference in specimen temperature profile.