Experiments were carried out on preparation of colloidal dispersions of ZnO nanoparticles in propylene glycol leading to ZnO-propylene glycol nanofluids. Thermal conductivity of these nanofluids was measured as a function of nanoparticle concentration (<= 2 vol.%), temperature (10-60 degrees C) and aggregate size. A strong dependence of thermal conductivity enhancement on temperature with higher enhancements at lower temperatures has been observed. Our results on temperature and aggregation dependence of thermal conductivity enhancement show that the thermal conductivity enhancement in ZnO-propylene glycol nanofluids is attributed to formation of solvation layers (liquid layers) of base fluid on the ZnO nanoparticle surfaces. (C) 2013 Elsevier B. V. All rights reserved.