A novel expression for the thermal conductivity of nanofluids is proposed, which incorporates the kinetic theory to describe the contribution of the Brownian motion of the nanoparticles with a more realistic definition of the mean free path, and additionally to consider the contribution of the interparticle interaction due to the existence of the electrical double layer (EDL). It is shown that this model is applied to Au/water nanofluids satisfactorily with respect to temperature, volume fraction and particle size. in the case of dense Al2O3/Water nanofluids, the effect of the interparticle interaction due to EDL on enhancing the thermal conductivity is more prominent than in the case of dilute Al2O3/water nanofluids. The model proposed in this paper shows that interparticle interaction due to EDL is the most responsible for the enhancement of thermal conductivity of nanofluids. (C) 2008 Elsevier Ltd. All rights reserved.