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1. Endorheic soda pans are among the aquatic systems that have the highest dissolved organic carbon (DOC) content on the planet with concentrations reaching values close to 1 g L−1. Considering the importance of inland waters in the global carbon cycle, the understanding of the drivers of such outstanding aquatic organic carbon pools is eminent. The soda pans of the Carpathian Basin present a wide variability of biotic and abiotic characteristics that provides an adequate system to assess the determinants of the extreme high DOC concentrations of soda pans. Here we demonstrate through a multisite comparison, a multiyear seasonal monitoring, and a laboratory experiment that the dissolved organic matter content of the highest DOC concentration soda pans is primarily of groundwater and emergent macrophyte origin. 2. More precisely, the multisite comparison of 14 soda pans revealed that the variation of colored dissolved organic matter (CDOM) content of the pans is partially explained by the CDOM content (21% of variation) and conductivity (14%) of the local groundwater indicating significant role of allochthonous terrestrial DOC sources. However, 46% of the variation in CDOM content of the studied soda pans could be accounted for the dominant type of emergent macrophyte with Bolboshoenus maritimus dominated macrophyte cover leading to higher CDOM content than Phragmites australis. 3. In line with the results of the multisite comparison, we demonstrated by a decomposition experiment that both B. maritimus and P. australis have the potential to release substantial amount of organic matter into soda pans. However, the organic matter release of B. maritimus is much more intensive than that of P. australis leading to twice as high DOC and 3.5-times higher CDOM concentrations. In general, considering previous organic matter release studies we concluded that P. australis is a relatively low organic matter releaser emergent macrophyte, and therefore the species composition of emergent macrophytes has to be considered in autochthonous plant-derived DOM estimations. 4. Finally, the multi-year seasonal monitoring of two distinctive soda pans showed that the high organic matter concentrations of the pans depends not only on their intrinsic characteristics but also on interannual variability. More precisely, we demonstrated that the highest CDOM and DOC concentrations occurred in a colored (i.e., brown, low TSS) soda pan that had extensive (95%) macrophyte cover dominated by B. maritimus in a period characterized by high pH due to low water levels, which were presumably the consequence of increased evaporation due to decreased precipitation and above average temperature. Considering the trends of climate change in Central-Europe (i.e., increased temperature and modified precipitation regimes), our results indicate that extremely high organic matter concentrations in soda pans might become more frequent in the near future.