'Therminol 55' is one of the heat transfer fluids used in process industries and possesses low freezing and high boiling points. The thermal conductivity of Therminol 55 can be improved through dispersion of nanoparticles leading to nanofluids. Magnesium oxide (MgO) nanoparticles (25-45 nm) were synthesized from magnesium nitrate by reaction with ammonium carbonate at 80 degrees C, followed by calcination at 800 degrees C for 1 h. MgO-Therminol 55 nanofluid was prepared by dispersing MgO nanoparticles in Therminol 55 using oleic acid as dispersant, followed by bath ultrasonication. Thermal conductivity ratio of nanofluids was found to vary linearly with nanoparticle volume fraction, while exhibiting biphasic trend with respect to the influence of temperature. Liquid layering and micro-convection caused by Brownian motion were found to be dominant contributors of thermal conductivity enhancement at lower (10 -30 degrees C) and higher (70-120 degrees C) temperatures respectively. Our data reveal linear increase in overall heat transfer coefficient with nanoparticle volume fraction under constant bath temperature condition, closely matching the thermal conductivity - nanoparticle volume fraction relationship. This is the first work on preparation, characterization and heat transfer performance of MgO-Therminol 55 nanofluids, highlighting its value for energy management. (C) 2015 Elsevier Ltd. All rights reserved.