The effects of atmospheric NH3 and SO2 separately and in combination on the growth and competitive ability of three species of the Violion caninae alliance were investigated. Growth and nutrient concentrations of Viola canina and Arnica montana in mixed culture with Agrostis capillaris were examined in relation to that in monoculture. Seedlings were transplanted into heathland topsoil in pots and placed in open-top chambers for 9 months from autumn to summer, where they were exposed to ambient air, 90 μ m−3 SO2, 50 μg m−3 NH3 and to the combination of NH2 and SO2. In the NH3+ SO2 treatment, a more-than-additive increase in nitrogen and sulphur concentrations was observed indicating co-deposition. NH3 influenced the nutritional status of V. canina the most, increasing the nitrogen, phosphorus, and magnesium concentrations and reducing those of potassium and calcium. NH3 fumigation significantly stimulated shoot growth of all three species and root growth of A. capillaris, while SO2 reduced only the root growth of A. capillaris. The relative yield of V. canina was reduced by 20–30% in the presence of the air pollutants. The relative yield of A. montana was stimulated by 30–40% in treatments including SO2 compared with that in ambient air or NH3 alone. The competitive ability of both dicotyledons in mixed culture with A. capillaris was strongly reduced by NH3 and was unaffected by SO2. The consequences of exposure to NH3 and SO2 for the survival and maintenance of threatened species in heathland vegetation are discussed.