Surfactant free, fairly stable ZnO-ethylene glycol (EG) nanofluids are prepared using prolonged sonication (>60 h). Extended period of sonication results in superior fragmentation and dispersion of ZnO nanoparticles, as is evident from the DLS data. Thermal conductivity is measured both as a function of ZnO nanoparticle concentration (0.5-3.75 vol%) and temperature (10-70 degrees C). A maximum thermal conductivity enhancement of similar to 40% (3.75 vol% of ZnO) is obtained at 30 degrees C which is substantially higher than that reported earlier on ZnO-EG nanofluids. All existing theoretical models fail to predict the present high thermal conductivity enhancement observed. A new expression for the thermal conductivity of nanofluids based on the contributions from the interfacial layer and the Brownian motion is proposed which explains the observed results fairly well. The effectiveness of the proposed expression has been further verified using the high thermal conductivity enhancement data reported earlier by several authors on other types of nanofluids. (C) 2012 Elsevier B.V. All rights reserved.