Increased cooling performance is eagerly required for many cutting edge engineering and industrial technologies. Nanofluids have attracted considerable interest due to their potential to enhance the thermal performance of conventional heat transfer fluids. However, heat transfer in nanofluids is a controversial research theme, since there is yet no conclusive answer to explain the underlying heat transfer mechanisms. This study investigates the physics behind the heat transfer behavior of Al2O3–H2O nanofluids under natural convection. A high spatial resolution flow velocimetry method – Particle Image Velocimetry – is employed in dilute nanofluids inside a Rayleigh-Benard configuration with appropriate optical access. The resulting mean velocity and flow structures of pure water and nanofluids are reported and their overall heat transfer performances are compared for Rayleigh numbers, Ra, of the order of 109. This paper aims to identify the contribution of the suspended nanoparticles on the heat and mass transfer mechanisms in low flow velocity applications, as those occurring during natural convection. The outcome of this work is a first step towards the evaluation of the applicability of nanofluids in applications where more complex heat transfer modes, namely boiling and Critical Heat Flux, are involved that are of great importance for the cooling of Fusion reactors.