Northern red oak (Quercus rubra L.) is one of the most important introduced tree species in the Czech Republic, occupying about 6,000 ha with ca. 900,000 m3 of the standing volume. The presented study aims to evaluate its soil forming effects on natural oak sites. Soil chemistry of the upper soil layers (F+H, Ah, B horizons) was studied in three pairs of stands of both species. In each stand, four bulk samples were taken separately for particular horizons, each consisting of 5 soil-borer cores. The soil characteristics analysed were: pH (active and potential), soil adsorption complex characteristics (content of bases, exchangeable cation capacity, base saturation), exchangeable acidity (exchangeable Al and H), total carbon and nitrogen content, and plant available nutrients content (P, K, Ca, Mg). Total macronutrient content (P, K, Ca, Mg) was analysed only in holorganic horizons. Results confirmed acidification effects of red oak on the upper forest soil layers such as decreased pH, base content, base saturation, all nutrient contents in total as well as plant-available form and increased soil exchangeable acidity (exchangeable Al) in comparison to the sessile oak stands, especially in holorganic horizons and in the uppermost mineral layer (Ah horizon). Northern red oak can be considered as a slightly site-soil degrading species in the studied sites and environmental conditions in comparison to native oak species.