Clay minerals are often associated with copper, gold and other valuable minerals and place a widespread problem in mineral flotation. This paper is one of a series addressing the problem caused by clay minerals in copper-gold flotation. This study seeks to understand the rheological behaviour of kaolinite and bentonite suspensions at natural pH and how the pH modifier, collector and frother normally used in copper-gold flotation affect the rheological properties. It was found that kaolinite and bentonite suspensions may follow Newtonian flows or non-Newtonian flows with pseudoplastic characteristics depending on the solid concentration. Bentonite has a stronger effect on the viscosity of suspensions than kaolinite. The pH modifier, collector and frother have a potential to alter the rheological behaviour of kaolinite and bentonite suspensions and the effect of the pH modifier is more pronounced. (C) 2013 Elsevier Ltd. All rights reserved.