The enhancement of the thermal conductivity of water in the presence of copper (Cu) using the chemical reduction method is presented in this study. It is the first time that the chemical reduction method for synthesis of nanofluids containing Cu nanoparticles in water is reported. No surfactant is employed as the dispersant. The volume concentration of Cu-water nanofluids is below 0.2 vol.%. Without the addition of dispersant and surfactant, the thermal conductivity of the produced nanofluids reveals a time-dependent characteristic. The thermal conductivity is the largest at the starting point of measurement and decreases considerably with elapsed time. The results show that Cu-water nanofluids with low concentration of nanoparticles have noticeably higher thermal conductivities than the water base fluid without Cu. For Cu nanoparticles at a volume fraction of 0.001 (0.1 vol.%), thermal conductivity was enhanced by up to 23.8%. (c) 2006 Elsevier Ltd. All rights reserved.