High concentrations of microplastics (MPs) found in marine ice rise questions on the behavior of MPs particles while water freezes, and on the resulting vertical distribution of the particles within the ice. We have performed (i) laboratory tests and (ii) sampling of sea-ice cores (RSF project No. 19-17-00041) to obtain first empirical insights.In laboratory, samples of fresh and salty (35 ppm) water containing various types of MPs particles were frozen. MPs were fibers (polyester), flakes (solid PS), and spherules (foamed PS) of different size fractions, both floating and sinking in the water of the sample. Physically, fresh-water freezes when water column is stably stratified, and the ice forms and grows over quiescent fluid. Fresh-water ice in the experiments was solid, transparent, containing only a few MPs inside, often captured by the air bubbles. Contrary, seawater is permanently mixed due to thermal convection until the very freezing, and then mixing is enhanced by the brine release. Sea-ice is eventually inhomogeneous (layered, with brine channels), not transparent, with much more MPs inside.Sea-ice cores (13 cores, total length of about 6 m) were collected with a ring drill (inner diameter 16 cm) at the section across the Amursky Bay (Sea of Japan) on February 21-27 2020, within flat areas of the ice field; CTD probe was used to measure parameters of the water layer beneath the ice. The state of the ice field and water layer under ice were monitored during the preceding period (30 days). In the lab, the cores were cut into layers of approx. 5 cm thick, weighed (700-900 g), and melted at room temperature. Finally, 122 filters were obtained and examined under microscope. Additional ice cores were taken for layer-by-layer (5 cm thick) determination of the integral salinity and dissolved organic matter. First results are obtained.