Ferric sulfates have attracted attention of geochemists since the discovery of jarosite on surface of Mars in 2004. To date, several ferric sulfates have been identified or suggested on Mars, including ferricopiapite, paracoquimbite, etc. These ferric sulfates, formed in evaporation or diagenetic processes, serve as mineralogical evidence for past water activity on Mars. More than proving the presence of water, ferric sulfates are hygroscopic and display complex hydration and dehydration transitions with change of air humidity and temperature, which suggest they may have application in tracing paleo-environment on Mars. On Earth, ferric sulfate minerals are environmentally important. Primarily distributed in acid mine drainage areas, ferric sulfates are hosts for various toxic metals and acidity. Their dissolution and precipitation greatly affect water quality and local environment. In this dissertation research, phase stabilities and transformations of ferric sulfates as functions of temperature (T), relative humidity (RH) and atmospheric pressure (P) were studied in detail. Techniques were developed and applied in this research to follow phase transitions of ferric sulfates in situ with changing environmental parameters and simultaneously collecting X-ray diffraction (XRD) data. Combined with parallel ex situ studies employing humidity buffer technique, phase relationships among ferric sulfates and the RH-T phase diagrams were delineated. Humidity and temperature effects on the evaporation of concentrated ferric sulfate solution were also investigated. Phase evolution sequences from concentrated ferric sulfate solution, including phases occurring in both precipitation and post-precipitation alteration processes, were characterized as a function of relative humidity, temperature and time. A typical evolution sequence, Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> solution → ferricopiapite and rhomboclase → kornelite → paracoquimbite, was identified at 40% to 60% RH at room temperature. An amorphous ferric sulfate formed from the Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> solution at RH < 30%. (H<sub>3</sub>O)Fe(SO<sub>4</sub>)<sub>2</sub> was identified as a dehydration product of rhomboclase, (H<sub>5</sub>O<sub>2</sub>)Fe(SO<sub>4</sub>)<sub>2</sub> · 2H<sub>2</sub>O, at low RH or high temperature. The phase boundary between these two compounds was resolved with in situ XRD studies at controlled RH and T and plotted in the RH-T phase diagram. Temperature is shown to be an important factor dictating ferric sulfate transition rates. The low temperature as present on Mars (-150 °C to 20 °C) could kinetically inhibit intermediate hydration phases, such as ferricopiapite and the amorphous ferric sulfate, from transforming to thermodynamically stable phases. This is why phase evolution maps resolved in this study are complementary to calculated phase diagrams. In situ T- and P-controlled XRD experiments showed ferric sulfates could be stabilized against variation of RH at T < 0 °C, indicating the diurnal RH variation (from almost 0 up to 100%) on the present day Martian surface can not affect hydration states of most ferric sulfates, except for a slight possibility to dehydrate ferricopiapite, which was found to be unstable at 6 °C and RH <1%.