This study summarises the rheological behaviour of emulsion bitumen drops in the presence of aqueous solutions of de-ionised or process water (DIW or PW) containing montmorillonite clays (M) and/or calcium ions (Ca++). The presence of calcium ions and montmorillonite clays resulted in the plastic behaviour of bitumen drops. In a DIW +M + Ca++ system, increasing temperature and calcium ion concentration resulted in an increase in the number and degree of plastic bitumen drops. In the presence of considerable amounts of Ca++ ions and/or at higher experimental temperature, bitumen drops in a PW + M system exhibited no significant overall plasticity of their surfaces. Both calcium and sodium ions contained in process water compete with each other to occupy the montmorillonite clay surface. At the pH value of process water (pH congruent to 8), increasing the temperature did not change the value of bitumen droplet zeta potential. Stability of bitumen-in-water emulsions at 22 degrees C showed that bitumen droplets coalesced upon contact in the DIW + M system. The addition of calcium ions (Ca++) led to the inhibition of coagulation and coalescence of bitumen droplets, which may indicate the formation of CaM aggregates at the bitumen-water interface. (c) 2004 Elsevier Inc. All rights reserved.