Carbon nanoparticles are very useful in solar thermal applications as they absorb much of the solar spectrum, are cheap and have excellent optical properties. Carbon nanoparticles-thermal oil-based nanofluid was prepared by a two-step method with diphenyl sulfone as the surfactant so that nano particles remained suspended even at high temperature. Particle size distribution was studied using two Dynamic Light Scattering systems at room and high temperature, which were also evaluated before and after exposing the nanofluid to thermal treatment so that conditions closer to those in real applications would be replicated. The morphological changes brought about by thermal treatment were observed by Transmission Electron Microscopy. Finally, optical properties, such as the ballistic transmittance, absorption coefficient and scattering albedo of both the base fluid and nanofluid, were measured by a spectrophotometer with and without integrating sphere. The results of this study contribute to knowledge about these solar nanofluids, which are promising alternatives to conventional solar collectors. (C) 2019 Elsevier Ltd. All rights reserved.