The work reports on the characterization of optical and dielectric properties of nanofluids consisting in pure nanodiamonds suspended in ethylene glycol. The study is carried out at four concentrations within the range from 0.01 to 0.1 wt%, and for two different types of nanodiamond powders, differing each other in the purity of the diamond phase (87% or 97%). Optical extinction, even at the investigated extremely low concentrations, results significantly increased in the colloids with respect to the pure base fluid, while the refractive index at 589.3 nm remains practically unchanged. Dielectric properties of nanofluids are related with the content of nanodiamonds particles. While the complex permittivity of tested samples is dependent on a small extent, the electrical conductivity is strongly affected by the increase of nanodiamond concentration. (C) 2019 Elsevier B.V. All rights reserved.