The use of hybrid nanofluids has been drawing attention of researchers in order to overcome the drawbacks of mono nanofluid and combine the physical and chemical properties of nanoparticles in a useful way. In the literature, a growing number of researches have been devoted to investigate the thermal performance of hybrid nanofluids. A significant amount of these researches are built on the theoretical correlations for the estimation of thermophysical properties of nanofluids. In the present study, a comparative study is conducted to reveal the influence of theoretical and experimental correlations on the heat transfer performance of hybrid nanofluids. Within this aim, theoretical and experimental based models for predicting thermal conductivity of hybrid nanofluid are evaluated by considering natural convection in a square cavity, which has been studied extensively in the literature. In the study, the natural convection of Al2O3/water, SiO2/water nanofluids and their hybrid combinations are investigated numerically for two different Rayleigh numbers (Ra = 10(4) and 10(5)) and three different particle volume fractions (phi = 1, 2 and 3%). The comparative analysis considering the studied parameters was performed in terms of local and mean Nusselt numbers. The results showed that employing theoretical models for thermal conductivity underestimates the heat transfer performance of both mono and hybrid nanofluids. Furthermore, it is surprising that the theoretically calculated SiO2/water nanofluid deteriorated the heat transfer performance. It was also observed that hybridizing the nanoparticles could perform the same heat transfer enhancement at a lower particle volume fraction compared to mono nanofluid (Al2O3). (C) 2019 Elsevier Ltd. All rights reserved.