Abstract: The stabilities of antlerite, Cu3SO4(OH)4, and a synthetic compound whose stoichiometry is here established as Cu3SO4(OH4·2H2O, have been determined at 5°C intervals between 10°C and 45°C using solution methods. The results of the experiments show that antlerite is stable with respect to the compound Cu3SO4(OH)4·2H2O only at temperatures above 30°C. Below 30°C a change in the relative stabilities of these two basic copper(II) sulfates occurs, and the compound Cu3SO4·(OH)4·2H2O, although unknown at present as a mineral, instead, is stable. Under these conditions, it does not react to give antlerite if kept in contact with water. Once isolated from its mother liquor, however, the dihydrate undergoes rapid dehydration to yield antlerite. Thermodynamic quantities for the two phases have been derived from equilibrium measurements. At 298.2K, values of ΔfG° for the compounds Cu3(SO4)(OH)4.2H2O(s) and antlerite are −1919.6(14) and −1445.0(10) kJ mol−1, respectively. The results have been used to construct stability field diagrams for the system CuO-H2O-SO3 at 25°C and at 35°C These diagrams have been used to illustrate the chemical conditions under which the two compounds might be expected to form in the oxidised zones of cupriferous base metal orebodies.