This dataset includes stream and rainfall hydrochemistry of the Plynlimon research catchments in Mid Wales. The data cover the period from March 2019 to March 2023. Sampling was carried out every four weeks, apart from a period of 5 months when access was restricted due to the Covid pandemic. 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.