Hydrothermal experiments at 400°C and 400 bars indicate that a complete solid solution exists between BaSO4and BaCrO4, whereas in the binary systems BaCrO4—SrCrO4, BaCrO4—PbCrO4, SrSO4—SrCrO4, PbSO4—PbCrO4and (at 100°C, 1 bar) BaSO4—Ba(BF4)2large miscibility gaps exist. Diagrams show the extent of the solid solutions in the systems (Ba, Pb) (SO4, CrO4) and (Ba, Sr) (SO4, CrO4). Stability calculations indicate that most chromates are too unstable to be common minerals. Natural lead chromates (crocoite) are almost free of substitutions, supporting the conclusion above.