This paper presents a multi-parameter investigation of Al2O3 nano-particles dispersed in distilled water, ethylene glycol and ethylene glycol-distilled water mixture (50/50 vol.%). The nano-fluid samples were prepared with ultrasonic processing with particle concentration up to 9 wt.%. Samples were characterized by transmission electron microscope imaging, particle surface charge (zeta potential), particle aggregate size, light absorbance and pH. The thermal conductivity and viscosity of the nano-fluid samples were measured. 10.9% enhancement in thermal conductivity was observed in ethylene glycol-distilled water mixture based nano-fluid with 5 wt.% Al2O3 nano-particle loading. The results show multiple perspective correlations between nano-fluid characteristics and thermal properties. The findings are summarized as follows. Combined effect of base fluid viscosity and particle surface charge determines the degree of nano-particle aggregation. Higher enhancement in thermal conductivity is observed with moderate particle aggregation, while either light or heavy aggregation lowers the enhancement. The ethylene glycol-water mixture based nano-fluids show overall higher thermal conductivity enhancement than the distilled water and ethylene glycol based nano-fluids. The degradation of nano-fluid thermal conductivity enhancement over time is found to be related to the gravity-driven particle sedimentation. Nano-fluids containing larger particle aggregates show higher viscosity augmentation. However, the extent of viscosity augmentation is not proportional to the extent of particle aggregation. (C) 2015 Elsevier Ltd. All rights reserved.