The objective of this study was to assess the effect of lime and soda ash as pH modifiers on the flotation of a copper-gold ore mixed with kaolinite at different concentrations. For the flotation experiments air flow rate, impeller speed, flotation time and other flotation reagents were kept constant, and the flotation outcomes were explained by the rheology behavior of the slurries and the type of particle aggregates formed with the addition of either lime or soda ash. Rheology measurements showed that lime increased apparent viscosities more than soda ash, but in Cryo-SEM images it was observed that the kaolinite aggregates in the slurry were similar in structure for both lime and soda ash additions. These two findings suggest that Ca2+ ions released from lime led to the formation of stronger aggregates that were more easily entrained as indicated by the changes in mass recovered to concentrate during flotation experiments. As a result, more dilution of the flotation concentrate grade occurred when adjusting pH with lime. A decrease in concentrate grade was the main negative effect in the flotation of the copper-gold ore mixed with kaolinite with the overall flotation recovery not affected. (C) 2015 Elsevier Ltd. All rights reserved.