Nanofluids display enhanced heat transfer properties, and several mechanisms, including Brownian motion and aggregation of the suspended particles, have been proposed to explain this enhancement. Agreement on a unified understanding of the mechanism has yet been reached. The role of particle aggregation is particularly controversial, and requires further study. Thus, this work analyzes the effects of particle aggregation on the thermal conductivity of alumina/water nanofluids. The thermal conductivity of the nanofluids was measured using the 3 omega method by varying the aggregation state until gelled nanofluids are formed. Brownian motion and sedimentation effects could thereby be removed from the analysis. The viscosity of the nanofluids was measured to quantify the aggregation state. The alumina/water nanofluids showed non-Newtonian characteristics in the low shear rate regime. The thermal conductivity could be explained by the classical Maxwell effective medium theory. High aggregation in the gel state produced large enhancements in the nanofluid thermal conductivity. (C) 2011 Elsevier B.V. All rights reserved.