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A large portion of terrestrially derived carbon outgasses as carbon dioxide (CO₂) from streams and rivers to the atmosphere. Particularly, the amount of CO₂ outgassing from small headwater streams is highly uncertain. Conservative estimates suggest that they contribute 36 % (i.e. 0.93 petagrams (Pg) C yr⁻¹) of total CO₂ outgassing from all fluvial ecosystems on the globe. In this study, stream <i>p</i>CO₂, dissolved inorganic carbon (DIC), and <i>δ</i>¹³C_DIC data were used to determine CO₂ outgassing from an acidic headwater stream in the Uhlířská catchment (Czech Republic). This stream drains a catchment with silicate bedrock. The applied stable isotope model is based on the principle that the ¹³C ∕ ¹²C ratio of its sources and the intensity of CO₂ outgassing control the isotope ratio of DIC in stream water. It avoids the use of the gas transfer velocity parameter (<i>k</i>), which is highly variable and mostly difficult to constrain. Model results indicate that CO₂ outgassing contributed more than 80 % to the annual stream inorganic carbon loss in the Uhlířská catchment. This translated to a CO₂ outgassing rate from the stream of 34.9 kg C m⁻² yr⁻¹ when normalised to the stream surface area. Large temporal variations with maximum values shortly before spring snowmelt and in summer emphasise the need for investigations at higher temporal resolution. We improved the model uncertainty by incorporating groundwater data to better constrain the isotope compositions of initial DIC. Due to the large global abundance of acidic, humic-rich headwaters, we underline the importance of this integral approach for global applications.