Abstract: Nanofluids are simply the suspensions of nanometer sized (< 100 nm) particles in various fluids. Because nanofluids have very high thermal conductivities compared to conventional base fluids, these fluids can find a wide range of applications such as automotive coolants, cooling advanced electronic packages, coolants in miniature heat exchangers etc. Various studies are being conducted to understand and utilize the multifaceted properties of nanofluids. The purpose of this experimental study is to evaluate the effects of several important parameters, such as particle species, surface charge, concentration, preparation techniques, and base fluids on thermal transport capability of nanoparticle suspensions (nanofluids). The surface charge was varied by changing the pH value of the fluids. The alumina (Al2 O3 ) and copper oxide (CuO) nanoparticles were dispersed in deionized (DI) water and ethylene glycol (EG), respectively. The nanofluids were prepared using both bath-type and probe sonicator under different power inputs. The experimental results were compared with the available experimental data as well as the predicted values obtained from Maxwell effective medium theory. It was found that that ethylene glycol is more suitable for nanofluids applications than DI water in terms of thermal conductivity improvement (up to 12% at 5% volume fraction loading) and stability of nanofluids. Surface charge can effectively improve the dispersion of nanoparticles by reducing the (agglomeration) particle size in base fluids. A nanofluid with high surface charge (low pH) has a higher thermal conductivity for a similar particle concentration. Sonication method and power have significant impact on the thermal conductivity enhancement. The collected results suggest that the key to the improvement of thermal conductivity of nanofluids is a uniform dispersion of nanoscale particles in a fluid.