A simple model makes it possible to relate the dielectric constant of spin-crossover nanoparticles to the indices of refraction and the absorption coefficients of solutions containing such nanoparticles. Using this model we show that in the visible spectral range, the switching from a diamagnetic (low spin state) to a paramagnetic (high spin state) state results in a noticeable change of the dielectric constants of the spin crossover nanoparticles containing 3, 5 and 7 iron (II) ions. Within the studied temperature range and for all the studied nanoparticles, the variation of their index of refraction is about 10%. (C) 2015 Elsevier B.V. All rights reserved.