Abstract: The optical effects of dichroism, static light scattering, and light attenuation under the action of a magnetic field in magnetic fluids with an average particle size of 6.7 and 13.7 nm were studied. Significant differences were found both in the magnitude of the effects and in their spectral behavior. They can be due to the formation of aggregates in the sample with larger particles, which significantly change the optical properties of the system. The formation of aggregates under the action of the field was confirmed by dynamic light scattering. © Pleiades Publishing, Ltd. 2024. ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2024, Vol. 88, No. 10, pp. 1518–1523. Pleiades Publishing, Ltd., 2024.