Red wood ant (Formica rufa group) mounds release high amounts of carbon dioxide (CO₂). As red wood ants and other invertebrates living in mounds are poikilothermal organisms, their metabolism and therefore CO₂ emissions are affected by changes in temperature. Thus, seasonal or diurnal changes in air temperature could affect CO₂ emissions from mounds. We found that seasonal mound CO₂ emissions and air temperature were correlated, both peaking in mid-summer. In contrast, diurnal CO₂ emissions and air temperature were inversely correlated, as we observed highest C fluxes during the night when air temperature was lowest. This CO₂ emission pattern can likely be explained by higher metabolic rates of ants resulting from their clustering, and increased numbers of ants in the mound when outside air temperature drops at night. Changes in microbial decomposition of mound organic matter or thermal convection of warm CO₂-rich mound air to the colder surface at night likely do not play a major role in the diurnal C fluxes observed in our study.