We examined the influence of Al₂O₃ and H₂O on the position of the coesite–stishovite transition by means of in situ X‑ray diffraction measurements with the large-volume press at the synchrotron PETRA III in Hamburg. The position of the transition was found to be shifted almost in parallel by about 1.5 GPa to lower pressures compared to results for the pure SiO₂ system by Ono et al. (2017). Stishovite of this study containing Al and H is only partially quenchable but transforms back to coesite.