Question: Can mixing ratio and species affect the use of substrate-derived CO2 by Sphagnum? Location: Poor fen in south Sweden and greenhouse in Wageningen, The Netherlands. Methods: Two mixing ratios of Sphagnum cuspidatum and S. magellanicum were exposed to two levels of CO2 by pumping CO2 enriched and non-enriched water through aquaria containing the species mixtures in the greenhouse. Results: Enhanced CO2 stimulated the production of S. cuspidatum, but only in aquaria co-dominated by S. magellanicum, coinciding with higher CO2 concentrations in the water layer. The denser growing S. magellanicum seemed to reduce gas escape from the water, resulting in accumulation of dissolved CO2. Adding CO2 did not affect species replacement. Conclusions: The use of substrate-derived CO2 for Sphagnum production depended on species identity and mixing ratio. The effect of mixing ratio on CO2 concentrations in the water layer suggests that species composition may affect both the efficiency with which substrate-derived CO2 is trapped and subsequently used. This could result in hitherto unexplored feedbacks between vegetation composition and gas exchange.