Nanofluids have attracted considerable attention in recent years as effective working fluids for heat transfer applications. This is not surprising given that nanofluids which are essentially suspensions of nanoparticles in a base fluid, exhibit higher thermal conductivity than conventional heat transfer fluids. The mechanisms responsible for such anomalous enhancement in thermal conductivity are still not well understood despite many experimental and theoretical investigations carried out on the subject. In this study, experiments were carried out on 70 nm alumina-in-water nanofluids with volume concentrations up to 13% in an attempt to develop a clearer understanding of the different mechanisms that could be responsible for enhancement of thermal conductivity in nanofluids. A set of experiments also were conducted on titanium dioxide-in-water nanofluids at volume concentrations of up to 5% to consider the effect of material on nanoconvection due to Brownian motion. Our findings indicate that nanoconvection caused by Brownian motion is the dominant mechanism responsible for the observed enhancements in thermal conductivity of nanofluids.