The well known variations in hydrothermal stability of VPI-5 with different heat treatment conditions were studied using argon porosimetry and X-ray diffraction (XRD) methods. This molecular sieve is extremely water-sensitive and transforms readily into AlPO 4 -8, even under mild conditions. For example, the conditions normally used simply for outgassing prior to adsorptive characterization drive this transformation. However, if the sample is dehydrated at low temperature in vacuum prior to heating, the subsequent heat treatments do not collapse the lattice. Furthermore, the AlPO 4 -8 that is produced adsorbs argon efficiently.