Subject of study. This study investigated the influence of the surface layer of nanoparticles on the refractive index of their hydrosols. Aim of study. The purpose of the study was to develop an optical method for investigating the surface layer of particles in liquid polydisperse systems and to test this method in the analysis of nanodiamond particles. Method. Relative changes in the refractive index and absolute changes in the system density with variations in the particle concentration were determined experimentally. Main results. Calculations based on Mie theory demonstrated that the refractive index of diamond hydrosols is weakly dependent on the size of the diamond particles, provided that their dimensions do not exceed one-fifth of the wavelength of the light used and their volume fraction remains unchanged. This enabled an extension of the applicability of the refraction theory for small particles that scatter light as dipoles. Diamond hydrosols with particles of various sizes were investigated, and the volume fraction of amorphous carbon on their surface was determined. Practical significance. The proposed method can be used to analyze the surface purity of nanodiamond particles during their production. © 2025 Elsevier B.V., All rights reserved.