The use of nanofluids as working fluids in direct absorption solar collector is growing up and the study of optical properties of nanoparticles is an important step for the success of this new technology.In this paper we report optical absorption measurements performed on several metal oxide nanoparticles (ZnO, CeO2, and Fe2O3) as a function of temperature in the range 25–500 °C, in order to study their optical properties, and to investigate how several heating cycles could affect nanoparticle structural stability and absorption characteristics. These are quite important issues to be investigated in order to assess the possibility to use such metal-oxide nanoparticles as gas-based high temperature nanofluid in Concentrated Solar Power (CSP).