One of the approaches to enhance the efficiency, and consequently, reduce costs to produce electricity from concentrated solar power (CSP) is by the development of advanced high temperature heat transfer fluids (HTFs). Incorporation of metallic nanoparticles into conventional heat transfer fluids could significantly improve the thermal transport properties of the HTFs. This study reports on the synthesis and investigation of copper nanoparticles synthesized in-house and dispersed in two synthetic HTFs Therminol 59 (TH59) and Therminol 66 (TH66). Liquid phase reduction of a copper salt was used to produce copper nanoparticles. Suspensions with various copper nanoparticle loadings (0.5-2 vol.%) were prepared. Characterizations such as the thermal conductivity, dynamic viscosity, mass specific heat capacity, and fluid stability were performed on the suspensions. Thermal conductivity enhancements over the base fluids were as high as approximately 20% at a 2 vol.% particle loading. These enhancements in the thermal conductivity are higher than the predictions based on the Effective Medium Theory (EMT). Dynamic viscosity measurements showed that if good dispersion of nanoparticles is achieved, the composite fluids behave in a Newtonian manner and the dynamic viscosity increases over the base fluid are minor at temperatures 125 C and above. Stability of the suspensions with time was also investigated. Based on the measured properties of the suspensions, a figure of merit for heat transfer was calculated to evaluate the viability of the suspensions. (C) 2014 Elsevier Ltd. All rights reserved.