The Lena River is one of the biggest Arctic rivers. Continuous permafrost covers about 72-80% of the drainage area, which will thaw with ongoing climate warming. Thus, the Lena potentially releases large quantities of permafrost-derived organic matter (OM) to the world’s largest continental shelf: the East Siberian Arctic Shelf. This OM consists of a complex mixture of compounds from different aquatic and terrigenous sources with different chemical/physical resistance towards decomposition and mineralization. Determining the sources (e.g., permafrost, soil, peat, phytoplankton, vegetation, etc.), quality and age of organic carbon (OC) transported by Arctic rivers is important to understand the effects of climate change on the river watersheds as well as on the Arctic coastal zone. Here we show results on composition and age of OM in Lena River. We collected the samples during the summer seasons of 2016, 2017 at the southern limit of the Lena Delta (Stolb Island). Water was sampled from three depths (surface, middle of stream and above the riverbed), two times per season. Stable carbon isotopes (δ13C) and radiocarbon values (Δ14C) were determined for particulate and dissolved organic carbon (POC and DOC, respectively). Our data show significant variations of POC isotopic composition between the studied years and within every season as well, while DOC isotopic values are more stable. POM appears to originate from older sources than DOM, as DO14C values are higher and indicative of more modern sources like recent terrestrial vegetation and aquatic primary production. Thus, isotopic survey illustrates spatial and temporal variability of POM composition and age in Lena Delta.