This dataset includes stream and rainfall hydrochemistry of the Plynlimon (Pumlumon in Welsh) research catchments in Mid Wales. The data cover the period from March 2024 to March 2025. Sampling was carried out every four weeks by a team of researchers from UK Centre for Ecology & Hydrology. Stream samples were taken using a grab technique and filtered in the field. Rain samples were collected using bulk precipitation collectors. Data are presented for major anions and cations, pH, conductivity, alkalinity and in-situ measurements for the water temperature and stream flow for the six stream locations and air temperature and volume for the rainfall sites. Full details about this dataset can be found at https://doi.org/10.5285/bb431417-01e8-43e7-bfe5-e48441b2e091
