This dataset contains water chemistry and organic matter composition variables. Water chemistry variables include pH, conductivity, dissolved oxygen, dissolved organic carbon, nutrients, anions, cations and metal concentrations. Organic matter composition variables include metrics calculated from carbon, nitrogen, hydrogen and oxygen concentrations. Water samples were collected from UK surface waters between 2018 and 2021. Water was collected from each site either annually, seasonally or monthly, to determine how organic matter varies over time and space. Site metadata include land use and vegetation cover, catchment area and percentage peat cover. Exact locations of sites are not included, and site names are anonymised. Full details about this dataset can be found at https://doi.org/10.5285/ada28810-040b-4fef-8669-b21bac64a10b