Summary: The chemical stabilities of mendipite, Pb₃O₂Cl₂, diaboleïte, Pb₂CuCl₂(OH)₄, chloroxiphite, Pb₃CuCl₂O₂(OH)₂, and cumengéite, Pb₁₉Cu₂₄Cl₄₂ (OH)₄₄, have been determined in aqueous solution at 298.2 K. Values of standard Gibbs free energy of formation, Δ Gf°, for the four minerals are −740, −1160, −1129, and −15163±20 kJ mol⁻¹ respectively. These values have been used to construct the stability diagram shown in fig. I which illustrates their relationships to each other and to the minerals cotunnite, PbCl₂, paralaurionite, PbOHCl, and litharge, PbO. This diagram shows that mendipite occupies a large stability field and should readily form from cold, aqueous, mineralizing solutions containing variable amounts of lead and chloride ions, and over a broad pH range. The formation of paralaurionite and of cotunnite requires a considerable increase in chloride ion concentration, although paralaurionite can crystallize under much less extreme conditions than cotunnite. The encroachment of the copper minerals on to the stability fields of those mineral phases containing lead(II) only is significant even at very low relative activities of cupric ion. Chloroxiphite has a large stability field, and at given concentrations of cupric ion, diaboleïte is stable at relatively high a_Cl⁻. Cumengéite will only form at high concentrations of chloride ion.