We report on laboratory experiments in which we allowed an SNC-analog mineral mixture to react with water under a simulated Mars atmosphere containing the added gases SO2, HCl and NO2. The addition of acidic gases was designed to simulate the atmospheric effects of periods of increased volcanic activity on Mars. The S/Cl ratio in our experimental brines was about 200, compared with an average value of approximately 5 measured in martian fines at the Mars Pathfinder and Viking 1 and 2 landing sites. This suggests that if salts were produced under an atmosphere higher in acidic volatiles than at present, they would have produced evaporite deposits higher in S than is presently found in the martian fines. Erosion of these high-sulfur evaporites and mixing with eroded mafic basalts could have produced the globally homogenous, salt-rich mobile fines that are present on Mars today.