The data includes phase boundary data among three ferric sulfates (kornelite, paracoquimbite, and ferricopiapite) under simulated Martian temperature and humidity conditions. The data includes: the adsorption water saturation curve data for the three ferric sulfates, individual Raman spectral data for each of the three ferric sulfates, typical spectral data for their binary mixtures, mass change data from phase boundary experiments, data of the phase boundary points, and Van 't Hoff curve data derived from the phase boundary data.