This dataset contains absorption coefficients of colored dissolved organic matter (CDOM) determined in surface waters of the Reloncaví Sound during the austral spring 2023–summer 2024 transition. Samples were collected at four stations along the estuarine gradient and stored at −20°C before spectrophotometric analysis. CDOM absorption spectra were measured following established optical methods (Nimptsch et al., 2014, 2015).