Nanofluids containing Fe3O4 and carbon nanotubes nanoparticles emulsified and dispersed using gum arabic (GA) and tetramethylammonium hydroxide (TMAH) were made and characterized for potential use as heat transfer fluids. Due to the interaction between the TMAH and GA molecules, the magnetic nanoparticles and CNTs were physically adsorbed. This paper reports an experimental work on the effect of ultrasonication on thermal conductivity of this aqueous suspension. The characterization and surface morphology of the dried samples were studied by using XRD and TEM measurements. Experiments were conducted in the magnetic nanoparticles mass concentration range 0.494-2.428%, CNT mass concentration range 0.0-1.535% and the temperature range 25-55 degrees C. Results show that thermal conductivity of the studied nanofluids is affected by ultrasonication time, increased first and then decreased after an optimum sonication time. Additionally, results show that addition of GA coated CNT nanofluid increases the thermal conductivity of the aqueous nanofluid containing TMAH coated Fe3O4 nanoparticles. (C) 2015 Elsevier B.V. All rights reserved.