Plynlimon is at the headwaters of the Rivers Severn and Wye. The infrastructure at Plynlimon consists of 10 instrumented research catchments representing different land use options on the same geological parent material. It is the UK’s Critical Zone Observatory, part of the EU and global network of CZOs and is an ExpeER site. The infrastructure records high temporal resolution river flow and meteorological data (40 year data record) and stream water and precipitation chemistry for cations, anions, trace metals, nutrients, pH and alkalinity (30 year data record) and some data for suspended sediment. The site has an extensive spatial data library including topography, digital terrain model (DTM), soils, geology and vegetation/land-use available, open access, through the Environmental Information Data Centre (https://catalogue.ceh.ac.uk/documents/91961a0f-3158-4d00-984d-91eb1e03e8bd). Sub sites in the catchments represent a range of upland management types: mature/clear cut/replanted conifer forest; acid grassland; eroding peatland; improved grassland.Samples analysed for a wide range of major, minor and trace components. Bulk precipitation and cloudwater chemistry is also measured at one location.