The thermal conductivity of nanofluids is known to be a function of the nanoparticle concentration and the particle size and is known to vary with time due to sedimentation and aggregation induced by particle-particle interaction. Therefore, it is crucial to measure the nanofluid thermal conductivity together with temporal variation of the volume fraction. However, researchers have reported only the estimation of the initial volume fraction based on the particle mass added to the base fluids and the primary particle size, which could be the crucial cause of the deviation between the thermal conductivity measurements from different research groups. In this study, a new method is introduced to track the temporal changes of the particle volume fraction and size distribution, and the effects of the change of the nanofluid thermal conductivity are scrutinized. Five different nanofluids are generated: DI water/Al2O3, DI water/SiO2, DI water/Ag, EG/Al2O3, and EG/ZnO. The particle volume fraction change is identified by monitoring the suspension density using a hydrometer, and the particle size distribution change is analyzed using the dynamic light scattering (DLS) method. The thermal conductivity of each of the nanofluids is measured using the transient hot-wire method. From the analysis of the data, the nanofluid thermal conductivity is found to be affected by the changes in concentration and the particle size distribution. (C) 2013 Elsevier Ltd. All rights reserved.