Abstract: The stability of synthetic connellite has been determined at 298.2K (25 °C) and 10⁵ Pa, using solution methods. For the reaction 1/37{62H⁺(aq) + Cu₃₇Cl₈(SO₄)₂(OH)₆₂.8H₂O(s) ⇌ Cu²⁺(aq) + 8Cl⁻(aq) + 2SO₄²⁻(aq) + 70H₂O(l)}, log K_H+ is equal to 6.44(2). This result has been used in turn to calculate a value for ΔfG°(1/37Cu₃₇Cl₈(SO₄)₂(OH)₆₂.8H₂O, s, 298.2K) of −423.7±6.6 kJ mol⁻¹. During the synthesis of connellite, claringbullite sometimes forms as a metastable phase. This solid recrystallizes to connellite if kept in contact with the reaction solution. The results have been used to construct an equilibrium model for the formation of connellite in relation to other common secondary copper (II) minerals. Connellite crystallizes from solution over an appreciable range of conditions. This result is consistent with the observed widespread occurrence of connellite, though as a very minor phase, in the oxidized zones of cupriferous sulfide ores.