Stable ZnO-water nanofluids with particle volume concentrations in the range of 0.25-2 vol% were prepared using probe ultrasonication and sodium hexametaphosphate for dispersion. The effect of temperature on hydrodynamic size distribution and zeta potential during heating and cooling cycle has been investigated to elucidate its role on dispersion characteristics. The study on influence of temperature on relative viscosity of ZnO-water nanofluids reveals temperature independency of relative viscosity upto a temperature of 35 degrees C and inverse temperature dependency in the temperature range of 35-55 degrees C. From the aspect of minimizing relative viscosity at a fixed nanoparticle concentration, excessive ultrasonication is unfavorable resulting in the formation of aggregates with lower fractal dimension. (C) 2012 Elsevier Ltd. All rights reserved.