Fluids containing nanometer-sized particles (nanofluids, NFs), are potential candidates to improve the performance and efficiency of several thermal devices at micro and macro scale levels. However, the problem of sedimentation and instability of these colloidal dispersions, has been the biggest obstacle for industrial scale applications. In this work, two different NFs were tested using distilled water (DI-Water) as the base fluid. The first is a traditional NF formed by Al₂O₃nanoparticles (NPs) with 50 nm of diameter and the second is a novel NF formed by poly (acrylic acid)-coated iron oxide NPs (Fe₃O₄@PAA) with ~10 nm of diameter, obtained through a hydrothermal synthesis process. The main objective of this study was to evaluate the colloidal stability of these NFs over time using different volume fractions and compare it with DI-Water. Results involving sedimentation studies and zeta potential measurements showed that the proposed Fe₃O₄@PAA NF presents a higher colloidal stability compared to that of the Al₂O₃ NF. Additionally, thermal conductivity measurements were performed in both Fe₃O₄@PAA and Al₂O₃ NFs at different NP concentrations, using the transient plane source technique. Results showed higher thermal conductivity values for the Fe₃O₄@PAA NFs compared to those of Al₂O₃ NFs. However, a linear enhancement of thermal conductivity with increasing NPs concentration was observed for the Al₂O₃ NF over the whole range of NP concentrations tested, whereas two different regimes were observed for the Fe₃O₄@PAA NF.