The Sommerodde positive organic carbon isotope excursion (SOCIE) within the Oktavites spiralis graptolite Biozone (Telychian, Silurian) was first identified in the Sommerodde-1 core, Bornholm, Denmark, where it is the largest positive excursion within the Upper Ordovician–lower Silurian part of the core. Other published occurrences of the SOCIE are discussed here, including new δ 13 C org data from the Jabalón River section, Corral de Calatrava, central Spain, where the SOCIE is only a very minor positive excursion. Very unusually, the SOCIE is best developed in deeper water settings, contrary to the typical pattern of declining excursion magnitude offshore. In the Sommerodde-1 core (Bornholm), and where it has been tentatively identified in the Vežaičiai-2 core (Lithuania), the SOCIE is developed in pale, organic-poor mudstones. It is considered likely that the magnitude of the SOCIE has been enhanced in the Sommerodde-1 core record by a change in organic matter composition in the deep-marine environment that did not have such a significant effect in shallower marine environments. Supplementary material: A table of organic carbon isotope data from the Jabalón River section, Corral de Calatrava, central Spain is available at https://doi.org/10.6084/m9.figshare.c.6769514 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system