This study reports two sets of dehydration experiments conducted on crystalline alunogen Al 2 (SO 4 ) 3 ·17H 2 O and melanterite FeSO 4 ·7H 2 O under Mars relevant pressure (P) and partial H 2 O pressure (P H2O ) at three temperatures (T). These experimental data are compared with those obtained from a similar set of experiments on crystalline epsomite MgSO 4 ·7H 2 O, and revealed the differences in the phase transition pathways of the dehydrations of these hydrous sulfates, with different cations and the highest hydration degrees, under Mars relevant conditions. Specifically, the differences in the dehydration rates were discussed, and further compared with the dehydration processes of Ca-sulfate and Fe 3+ -sulfate of the highest hydration degrees. We found that the dehydrations of Mg-sulfate and Fe 2+ -sulfate would progress the fastest on Mars.