A promising new generation of solar thermal collector able to enhance the thermal efficiency is the DASC (Direct Absorber Solar Collector). In this paper we report optical absorption measurements performed on several water-based nanofluids (Al2O3, CuO, TiO2, ZnO, CeO2, and Fe2O3) as a function of nanoparticles concentration. These measurements are of fundamental importance to assess the possibility to use the above mentioned metal-oxide nanoparticles in liquid-based nanofluids for direct absorption low temperature flat panel solar collector. The obtained results show different optical behaviors of the nanofluids depending on nanoparticles material and concentration. In all measurements the transmittance rises passing from visible to infrared region and in some cases, when the nanoparticles concentration is too low, the extinction distance grows up to values larger than the typical diameter of a solar …