To clarify the nutrient dynamics in peat-covered watersheds during frost, soil, chemical properties of atmospheric deposition (bulk deposition and throughfall) on mires, peat pore water, and stream-water outflows from mires were investigated at three ombrogenous mires with Picea glehnii M. forests and Sphagnum spp. communities in Ochiishi, eastern Hokkaido, Japan. We investigated the depth of frozen ground as one of the factors determining chemical properties of outflowing stream water from mires. Na and Cl⁻ were the dominant ion species in bulk deposition and throughfall, implying the influence of sea salts on precipitation chemistry. The electrical conductivity (EC) and Na , Mg² Ca² , Cl⁻, and SO₄²⁻ concentrations in throughfall (snow) were 5–10 times higher than bulk deposition after a 25-day period without precipitation in February 2001. Throughfall was enriched with sea salts from dry deposition on the canopy, while snow was intercepted by the forest canopy. The first rain event during the seasonal soil freezing led to the enrichment of chemical components in the stream-water outflows from the mire. The frozen layer in the upper peat soil prevented the infiltration of snow melt into peat soil during the period of frost soil, and hence large amounts of salts (Na and Cl⁻) accumulated on the snow surface or within the snow cover. Rain water flowed over the frost soil layer and was enriched with chemicals from accumulated salts in the snow cover. This subsequently led to the high salt concentration in stream water just after the rain events during the season when soil was frozen.