Heat transfer characteristics of dilute nanofluids under natural convection regime have been experimentally studied. The buoyancy-driven convection has been set-up in a differentially heated cavity containing basefluid (de-ionized water) and Al2O3/water-based dilute nanofluids. Temperature difference has been employed across the two horizontal walls of cavity, the side walls being thermally insulated. Experiments have been performed for four volumetric concentrations (0.01%, 0.02%, 0.03% and 0.04%) of nanofluids. Projection data of the temperature field has been recorded using a Mach-Zehnder interferometer. Results have been presented in the form of interferograms of the convective field, two-dimensional distributions and variations of heat transfer coefficients as a function of nanofluid concentrations and temperature differences. Results of the study reveal a strong dependence of convective field on the concentration of nanofluids. At low Rayleigh numbers, while the heat transfer through central core region of the cavity was predominately through conduction for de-ionized water, significant convective effects were seen in the case of nanofluids. At higher values of driving potential, strong convective movements in the entire fluid layer were observed irrespective of the type of fluid employed. In nanofluid based experiments, well-defined roll-like convective structures were to be seen with an increase in cavity temperature difference. Time-dependent movement of these dominant structures was observed. Spectral analysis of the intensity-time signal revealed an increasing trend in the frequency of oscillation with an increase in the concentration of nanofluid for any given value of AT. The experiments revealed that at higher driving potential, with increasing concentration of nanoparticles, the bigger structures continuously form and break into smaller roll-like structures, thus indicating better mixing in fluid layer, leading to higher heat transfer rates. (C) 2015 Elsevier Ltd. All rights reserved.