In flotation of copper sulphide minerals with alkyl xanthate collectors, the hydrophobic coating formed on mineral surface contains co-adsorbed cuprous xanthate and dixanthogen. In addition to these adsorbed species, copper xanthate precipitates are likely to form in the bulk of the flotation liquid whenever copper ions released into solution (during grinding preceding the addition of a collector) from the dissolution of surface oxidation products encounter xanthate ions. The objective of the present study was to find out how such precipitates form in a homogeneous liquid system and, once formed, how stable they are under acid or alkaline conditions at temperatures varying from 20° to 60°C. Mixing solutions of potassium ethyl xanthate and cupric sulphate, of various concentrations, has established that: 1. (a) when near-equivalent concentrations of both reactants are used, a precipitate (either a coarse granular one—if high concentrations in excess of 10 −3 M are used, or a finely dispersed sol) is formed provided that the product of concentrations exceeds the solubility product by 2–3 orders of magnitude. 2. (b) if nonequivalent concentrations are used, mixing results in transparent solutions even if the product of concentrations exceeds the solubility product by 5–6 orders of magnitude. Ionized complex species, Cu(X) + [where X denotes xanthate ion, ROCSS −], absorbing at 284 nm, appears to be the initial result of interactions between the two reactants. The granular copper precipitate is a stoichiometric mixture of cuprous xanthate and dixanthogen, and not—as suggested by Solozhenkin (12), following his EPR study—a cupric xanthate. Application of ESCA technique has shown clearly that the appearance of cupric form (due to unavoidable superficial oxidation on handling the samples) masks in the EPR the predominant cuprous form of the bulk of the sample under investigation. The stability of the copper precipitate in an open system exposed to atmosphere (as in a flotation cell) has been investigated at temperatures 20°C, 35°, 45°, and 60°C, in acid and alkaline solutions ranging from pH 4.5 to 12.5. Decomposition products, detected by different techniques, were found to consist of insoluble solids (hydroxide, carbonate, or sulphide) and dissolved thin-compounds, xanthate ions, CS2, and alcohol. The stability of copper xanthate precipitate is closely related to the stability of adsorbed coating on copper sulphide minerals and has important implications in the development of selective separation procedures for minerals that have been initially floated together as a bulk concentrate.