Abstract: New stability constant data are presented for the minerals blixite, mendipite and the compound Pb7O6Cl2.2H2O at 298.2 K and P = 105 Pa. Mendipite is in fact a metastable phase at this temperature, being thermodynamically stable under the appropriate conditions at temperatures above about 29°C. Kinetic influences are of some significance with respect to the sequence of formation of solid phases in the PbO-HCl-H2O system, and these have been elucidated for some important reactions. Penfieldite and fiedlerite appear to be metastable phases at all temperatures at 105 Pa. The results have been used to reassess the conditions of formation of the lead(II) oxy- and hydroxychloride phases that are known to form as minerals and as corrosion products of lead-containing artefacts. The effect of CO2 on the system is also described.