Summary We studied the effects of elevated atmospheric CO 2 and increased N deposition on the plant species composition of a Sphagnum ‐dominated bog ecosystem in the Netherlands. Large peat monoliths (surface area 1 m 2 , depth 0.6 m) with intact bog vegetation were kept outdoors in large containers and were exposed to elevated CO 2 or increased N deposition for three growing seasons. Elevated CO 2 conditions (target concentration 560 µmol CO 2 mol −1 ) were created using MiniFACE technology. In a separate experiment, N deposition was increased by 5 g N m −2 year −1 by adding dissolved NH 4 NO 3 at 3 week intervals during the growing season. Elevated atmospheric CO 2 increased height growth of Sphagnum magellanicum , the dominant Sphagnum species, in the second and third growing seasons. Vascular plant biomass was not significantly affected by elevated CO 2 , but growth of species growing close to the moss surface was influenced negatively by the increased Sphagnum height growth. Elevated CO 2 did not change allocation to below‐ground plant parts. Adding N increased above‐ground vascular plant biomass. The shallow‐rooted species Vaccinium oxycoccus responded most to the increased N deposition. Sphagnum growth was significantly reduced in the third growing season. This reduction was likely the result of the increased vascular plant cover, given the observed negative relation between vascular plant cover and Sphagnum growth. The observed shifts in species composition as a result of species‐specific responses to treatments, and interactions between peat mosses and vascular plants will have important consequences for the sequestration of carbon in the bog ecosystem.