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(1) Molinia caerulea occurs in peatland vegetation communities. However, under certain environmental and management conditions it may overwhelm the typical peatland vegetation, compromising ecosystem services including carbon dioxide (CO2) sequestration and storage. Improved understanding of the effects of M. caerulea on CO2 fluxes in peatland ecosystems will better inform landscape management. (2) Photosynthesis at a PPFD of 600 μmol m 2 s-1 (PG600), ecosystem respiration and partitioned (heterotrophic and autotrophic) below-ground respiration were calculated from closed-chamber measurements collected over the 2012–2014 growing seasons within two drained M. caerulea dominated peatland catchments in Exmoor National Park, southwest England. (3) All CO2 fluxes peaked in mid-summer, in association with increased soil temperature and vegetation growth. During wetter periods, below-ground autotrophic respiration and PG600 significantly decreased (p = 0.028 and p = 0.002), conversely heterotrophic respiration increased insignificantly (p = 0.081). An empirically derived net ecosystem exchange model estimated the growing season of 2012 to have been a smaller CO2 sink in carbon equivalents ( 126 ± 243 g m 2) than subsequent growing seasons ( 146 ± 310 and 234 ± 325 g m-2) due to lower photosynthetic rates during a cool, wet summer (2012). (4) Management aimed at preserving the carbon store and increasing carbon sequestration through raising water tables may reduce photosynthesis and increase heterotrophic respiration unless the dominance of M. caerulea is reduced and Sphagnum spp. cover increased.