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This paper reports an experimental study on the natural convective heat transfer of nanofluids, an area in which little work has been carried out in the past. Aqueous-based titanium-dioxide nanofluids of various concentrations are formulated by using the two-step method and a high shear homogenizer is used to break large aggregates. Instead of the use of dispersant and/or surfactant, the electrostatic repulsion mechanism is adopted to stabilize nanoparticles. The resulting nanofluids are found to be very stable, although the actual measured particle size is much larger than the primary nanoparticle size. The stable nanofluids are then used for both the transient and steady-state heat transfer experiments under natural convection conditions. The results show that the presence of nanoparticles systematically decreases the natural convective heat transfer coefficient under the conditions of this study, which is an observation that contrasts with the previous expectation. Discussion of the results suggests that changes in the nanofluids' thermal conductivity and viscosity could not explain the observed decrease in the heat transfer coefficient, and particle-surface interactions may play an important role.