It has been proposed that blending of the Kennecott Utah Copper Corporation (KUCC) Bingham Canyon poorly-floating limestone skarn (LSN) ore with the monzonite ore (MZ), which in isolation floats well, results in a 'poisoning' effect on chalcopyrite, bornite and molybdenite flotation recovery. This proposition has been examined via a combined study of mineralogy, liberation, solution, surface and SEM analyses of batch flotation samples. Cu grade versus recovery for the blended ore (70% MZ: 30% LSN) was close to that predicted by pro-rata flotation of the separate fractions and hence flotation poisoning of the MZ copper containing minerals is not occurring. In contrast recovery and grade of molybdenite, which is only present in significant quantities in the MZ ore, was significantly reduced as compared to pro-rata prediction, indicative of a poisoning effect on flotation performance. Although pre- and post-conditioning and tailings water samples were super-saturated with respect to a range of Mg and Ca-containing species similar supersaturation trends were observed regardless of the ore or blend used. Hence, precipitation of hydrophilic species onto mineral surfaces cannot be used to explain the differences in flotation phenomena observed. Similar surface contamination of chalcopyrite, bornite and molybdenite in both the LSN and blended ore concentrates was observed, whereas the surfaces of molybdenite in the MZ concentrates were relatively clean. The principal surface contamination of the concentrates for the blended ore, which increased in severity between the 1st and 4th concentrates, was from Ca and Fe hydroxides, phosphates and Ca and Mg aluminosilicate gangue. This is in agreement with the observed mineralogy with the LSN ore containing considerably greater proportions of the Mg-containing minerals pyroxene. amphibole and talc, and the Ca-containing minerals, calcite and andradite than the MZ ore. It is suggested that the transfer of hydrophilic components from the LSN to MZ ores on blending has little effect on Cu mineral flotation due to these minerals already having similar levels of surface contamination within the two ore types. However, the formerly relatively clean molybdenite surfaces from the MZ ore are contaminated as a result of blending leading to a dramatically reduced flotation response. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.