This dataset presents the loss-on-ignition analysis results of sediment cores collected in the Trunvel and Kerallé valleys, Brittany, North-West France. These analyses were carried out as part of the doctoral project Paleogeographic changes and coastal archaeological remains in Brittany during the Middle and Late Holocene: Contributions of a multidisciplinary and multi-scaled approach, with the aim of reconstructing the Holocene evolution of small estuarine systems on the North-West French coast.Loss-on-ignition (LOI) measurements were carried out on approximately 1 g of sediment sampled at 2 cm intervals from cores TRU-C1 and ANE-C3. After drying at 105°C for 16 hours, the samples were heated to 550°C for 4 hours to estimate their organic matter content. A second heating phase, to 950°C for a further 2 hours, was undertaken to assess the proportion of carbonates. Samples were weighed at each stage, and sediment mass loss was expressed as a percentage.