Iron is an essential tracer of the evolution ofaqueous environments, redox conditions, and habitability on theMartian surface. At present, there is still a lack of constraints on theinitial FeII/FeT ratios of Martian brines and how evaporative FeII−FeIII sulfate brines evolve under a CO2 atmosphere and ultraviolet(UV) irradiation. In this study, we experimentally investigated theevaporation of Fe sulfate brines (initial FeII/FeT ratios from 1 to 0;pH ≤ 2.8) under CO2, UV/CO2, and UV/Earth conditions. Ourresults show that rapid dehydration of acidic FeII−FeIII sulfatebrines produces FeII- and FeIII-sulfates separately. Rozenite is theprimary FeII-sulfate phase in most cases and is usually associatedwith szomolnokite, which forms via rozenite dehydration. FeIIsulfate evaporites show a clear evolution pathway of dehydrationfrom 7w or 4w to lower hydration states. Rhomboclase−ferricopiapite mixtures are primary FeIII phases, and kornelite is a minorphase occurring only when RH > 30%. Oxidization of FeII-sulfate brines produces ferrihydrite under UV/CO2 conditions and amixture of schwertmannite, ferrihydrite, hydronium jarosite, and hematite under UV/Earth conditions. Acidic and concentratedFeII−FeIII sulfate brines can still be oxidized on present Mars. Evaporative and oxidative products show better crystallinity andvariety under stronger oxidizing conditions. FeIII phases may be important contributors to amorphous materials detected on Mars,not only in the form of nanocrystalline Fe oxides (e.g., ferrihydrite and hematite) but also nanocrystalline sulfates (e.g.,ferricopipiate) and colloids by FeIII hydrolysis. FeIII phases are good reservoirs of H2O in general, and their lack of furthercrystallization may indicate a persistent cold environment on Mars.