The solubility at 25 °C. of the sparingly soluble basic copper carbonate malachite has been determined by two methods. The constancy of the calculated value of the solubility product in solutions of varying ionic strength has shown it to be a comparatively well-defined material suitable for the study of the complexing ability of various organic chelating compounds. Evidence is presented for the existence of a cupric ion – carbonate ion complex under appropriate pH conditions.