The aim of this study is to explore the anomalous variation of thermo-physical properties of aqueous nanofluids. The specific heat of three water-based nanofluids containing silicon dioxide (SiO2), titanium dioxide (TiO2), and aluminum oxide (Al2 O3) nanoparticles were measured using a differential scanning calorimeter (DSC). Measurements were performed over a temperature range of 30°C - 80°C which was chosen to be between melting point and boiling point of water. The experiments were implemented with different sizes of nanoparticles to investigate the effect of the-size of nanoparticles on the specific heat of nanofluids. The specific heat of the nanofluids was plotted as a function of the diameter of nanoparticles and the mass concentration of nanoparticles. The results indicate that the specific heat of aqueous nanofluids decreases as the mass concentration of nanoparticles increases from 0.5 % to 20 %. Moreover, the results show that the specific heat of nanofluids is less sensitive to the variation of the nanoparticle size and more sensitive to the variation of the mass concentration of nanoparticles. A simple analytical model for the specific heat of nanoparticle suspensions in a solvent is proposed to explain the observed behavior. The model accounts for the contribution to the specific heat by an interfacial layer formed at the solid-liquid interface. The predictions from the proposed analytical model for the specific heat of nanofluids are found to be in close agreement with the experimental results.