Natural malachite is a well defined solid demonstrating reproducible solubility behavior over a wide range of pH. The following equilibrium constants associated with the malachite dissolution equilibrium at 25°C, 1 atm were determined: K sp = a 2cu 2+a CO 32-K 2w/a 2H+ = 3.5 ± 0.6 × 10 -34(infinite dilution) K ∗sp = [Cu 2+] 2[CO 2-3]K 2w/a 2H+ = 10. ± 0.2 × 10 -32 (0.72 ionic strength) K' sp = m 2Cu 2+m COsu2-3K 2w/a 2H+ = 1.3 ± 0.1 × 10 -28 (36.9‰ salinity seawater). The temperature dependence of a "mixed" equilibrium constant, Ksp+, of the form: K †sp = [Cu 2+] 2m CO 2-3K 2w/a 2H+ has been measured at I = 0.72, yielding the relationship: log K †sp = (- 9.8 ± 0.03) × 10 4( 1/T°K) + (1.52 ± 0.09) within a 5-25°C temperature range. The effect of pressure on the solubility of malachite in water and seawater was estimated from partial molar volume and compressibility data. For 25 °C at infinite dilution K' sp (1000 bar) /K' sp(0) = 240 and in seawater K' sp(1000) /K' sp(0) = 44 . Comparison of stoichiometric and apparent malachite equilibrium constants has been used to estimate the extent of copper(II) ion interaction at the ionic strength of seawater. In dilute carbonate medium (total alkalinity, TA = 2.4 meq/kg H2O, pH 8.3), 2.9% of total dissolved copper exists as the free copper(II) ion and in seawater ( S = 36.9%., TA = 2.3 meq/kg H2O, pH = 8.1), [Cu 2+] /T(Cu) is 3.1%. Total dissolved copper levels of approximately 450-750 nMol/Kg are necessary to attain malachite saturation conditions in the open ocean. Observations of malachite particles suspended in seawater must be explained by precipitation or solid phase substitution reactions from localized environments rather than by direct precipitation from bulk seawater.