Combined experimental, modeling, and analytical results indicate that the rapid acidification of dilute waters in contact with nominally Fe2+-sulfate minerals (FeSO4nH2O) is caused by Fe3+ hydrolysis, which occurs when low levels (<1 mol%) of a contaminant Fe3+-sulfate phase are dissolved along with the FeSO4nH2O. This rapid acidification has previously been attributed to hydrolysis by Fe2+. However, dissolution experiments performed using ZnSO4nH2O, in which the Zn2+ cation has a higher hydrolysis constant (log K = −8.96) than Fe2+ (log K = −9.5), failed to produce significant changes in solution pH. We present the results of geochemical modeling simulations confirming that FeSO4nH2O dissolution alone cannot explain the experimentally observed change in pH from 5.65 to 3.50. Nor can the experimental observations be explained by oxidation of Fe2+ to Fe3+ in solution. Instead, our experimental results can be best explained by modeling the incorporation of <1 mol% Fe3+ contamination from any number of Fe3+ or mixed valence Fe-sulfate phases, including anhydrous Fe23+ (SO4)3, coquimbite, kornelite, römerite, bilinite, copiapite, or ferricopiapite, all of which are reasonable candidate phases for oxidative breakdown products of FeSO4nH2O. Laboratory Mössbauer spectra are consistent with up to 0.6 mol% of the total Fe in the sample to be present as Fe3+. Although the doublet has parameters that are not diagnostic of any specific Fe3+-sulfate, they do help constrain its identification. These results demonstrate that minor contamination of labile Fe2+ sulfates by Fe3+ can have dramatic effects on solution chemistry that should be considered when studying reactions relevant to acid mine drainage waste sites and other localities where Fe-sulfate minerals occur, such as the surface of Mars.